COMMUNICATION SYSTEM Technical Field The present invention relates to mobile communications devices and networks, particularly but not exclusively those operating according to the 3 Generation Partnership Project (3GPP) standards or equivalents or derivatives thereof. The invention has particular although not exclusive relevance to the Long Term Evolution (LTE) of UTRAN (called Evolved Universal Terrestrial Radio Access Network (E-UTRAN)). Background Art In 3GPP LTE networks, a base station (i.e. evolved NodeB, eNB) of a Radio Access Network (RAN) transmits data and signalling between a core network (CN) and User Equipment (UEs) located within the base station's coverage area. Developments in communication networks have seen increased deployment of so called 'small' cells operated by Low Power Nodes (LPNs), such as pico eNBs, femto cells, Home eNBs (HeNBs) or the like, which cells have a smaller coverage area than existing macro cells operated by a higher power macro base station. Networks comprising a number of different cell types, for example a network comprising a macro cell and a femto cell, are referred to as Heterogeneous Networks, or HetNets. More recently the need to make further enhancements to small cells using low-power nodes, has been identified as one of the most important topics for further development of 3 GPP standards compliant communication systems in order to enable such communication systems to cope with increases in mobile traffic especially for hotspot deployments in indoor and outdoor scenarios. According to this interest in small cell enhancements, scenarios and requirements for small cell enhancements were studied and captured in a 3GPP technical report (3GPP TR 36.932) the contents of which are herein incorporated by reference. TR 36.932 defines a low-power node as generally meaning a node whose transmit power is lower than that of macro node and base station classes. For example, as indicated above, both pico eNB and femto HeNBs are considered to be low power nodes. Currently, the average geographic density of macro cells is 5-7 macro cells per square kilometre. However, it is predicted that the number of small cells in urban areas will reach 40 3 small cells per square kilometre. The increasing geographic density of small cells presents challenges in achieving successful and efficient handover procedures in HetNets. In a densely deployed small cell scenario for example, the number of small cells a piece of user equipment (UE) such as a mobile ('cellular') telephone or other similar mobile device should detect and measure for the purposes of mobility management (e.g. management of the smooth transfer from one cell to another) can be much larger than that of other macro cell or less dense HetNet scenarios. Accordingly whilst, in the conventional LTE systems up to the latest Release 11, the use of Synchronisation Channels (SCH) which consist of a Primary Synchronisation Channel (P-SCH) and a Secondary Synchronisation Channel (S-SCH) can be used to help the UE to search, acquire, and synchronise with an eNB, it is generally accepted that the SCHs will be unable to cope with such a densely populated cell deployment scenario. Specifically, there are a number of other issues that need to be considered for a densely populated small cell environment including, for example: how to ensure efficient use of power; the speed of the search for/discovery of a small cell; the ability to identify the cells; and/or the ability to acquire measurement results for the purposes of handover. Summary of Invention Accordingly, preferred embodiments of the present invention aim to provide methods and apparatus which overcome or at least alleviate one or more of the above issues. In one aspect of the invention there is provided communication apparatus for communicating with a mobile communication device in a communication system which uses a plurality of radio frames wherein each radio frame is subdivided in the time domain into a plurality of subframes, each subframe is subdivided in the time domain into a plurality of slots, and each slot is subdivided in the time domain into a plurality of symbols, said communication apparatus comprising: cell operating means for operating a communication cell; discovery signal generating means for generating discovery signals, for use in a cell search procedure, each discovery signal comprising a pair of synchronisation signals and at least one further signal; and transmitting means for transmitting each of said pair of synchronisation signals and said at least one further signal in a respective symbol of a radio frame; wherein said symbol in which said at least one further signal is transmitted separated, in the time domain, by no more than half a radio frame from at least one of said pair of synchronisation signals. The transmitting means may be operable to transmit said at least one further signal in a different subframe to said pair of synchronisation signals. 4 The at least one further signal may comprise a plurality of further signals and wherein said transmitting means may be operable to transmit each of said plurality of further signals in a different respective subframe. The transmitting means may be operable to transmit said at least one further signal in the same subframe as said pair of synchronisation signals. The transmitting means may be operable to transmit said at least one further signal in a different slot to said pair of synchronisation signals. The transmitting means may be operable to transmit said at least one further signal at a separation, in the time domain, of a single slot from one of said pair of synchronisation signals. The transmitting means may be operable to transmit the, or at least one, further signal in a symbol that is adjacent, in the time domain, to the symbol in which one of said pair of synchronisation signals is transmitted. The at least one further signal may comprise a plurality of further signals and wherein said transmitting means may be operable to transmit each of said plurality of further signals in a symbol that is adjacent at least one other symbol in which one of said plurality of further signals is transmitted. The pair of synchronisation signals may comprise at least one of a primary synchronisation signal (PSS) and a secondary synchronisation signal (SSS). The pair of synchronisation signals may comprise a primary synchronisation signal (PSS) and a secondary synchronisation signal (SSS). The at least one further signal may comprise at least one of a primary synchronisation signal (PSS) and a secondary synchronisation signal (SSS). The at least one further signal may comprise a primary synchronisation signal (PSS) and a secondary synchronisation signal (SSS). The at least one further signal may comprise at least one signal that is different to a primary synchronisation signal (PSS) and/or a secondary synchronisation signal (SSS). The cell operating means may be operable to operate said cell as one cell of a cluster of cells each cell of which shares a common cell identifier (or 'cluster identifier'); said discovery signal generating means is operable to encode an identifier for said one cell into at least one of said pair of synchronisation signals and said at least one further signal; and wherein said identifier for said one cell is unique at least within said cluster of cells. The discovery signal generating means may be operable to encode said common cell identifier (or 'cluster identifier') into at least one of said pair of synchronisation signals and said identifier for said one cell into said at least one further signal. 5 The discovery signal generating means may be operable to encode said common cell identifier (or 'cluster identifier') into said pair of synchronisation signals by encoding a cell group identifier into a first of said pair of synchronisation signals and a physical layer cell identity (PCI), identifying the cell within a cell group represented by the cell group identifier, into a second of said pair of synchronisation signals. The identifier for said one cell, said cell group identifier, and said PCI together may provide a global identifier for said one cell. The discovery signal generating means may be operable to encode said identifier for said one cell, as a global identifier for said one cell that does not depend on the common cell identifier, into at least one of said pair of synchronisation signals and said at least one further signal. Each of said radio frame may be divided, in the frequency domain into a plurality of resource blocks, each resource block being divided, in the frequency domain, into a plurality of sub-carriers, wherein at least one of said pair of synchronisation signals and said at least one further signal may extend across more than six resource blocks in the frequency domain. The at least one of said pair of synchronisation signals and said at least one further signal may extend into at least twelve resource blocks in the frequency domain. According to another aspect of the invention there is provided a communication apparatus for communicating with a mobile communication device in a communication system which uses a plurality of radio frames wherein each radio frame is subdivided in the time domain into a plurality of subframes, each subframe is subdivided in the time domain into a plurality of slots, and each slot is subdivided in the time domain into a plurality of symbols, said communication apparatus comprising: means for operating a communication cell as one cell of a cluster of cells each cell of which shares a common cell group identifier, and a common physical cell identity (PCI), which cell group identifier and common PCI together form a shared common cell identifier (or 'cluster identifier'); discovery signal generating means for generating discovery signals, for use in a cell search procedure, each discovery signal comprising at least a pair of synchronisation signals and encoding an identifier for said one cell, wherein said identifier for said one cell is unique at least within said cluster of cells; and means for transmitting each of said pair of synchronisation signals in a respective symbol of a radio frame. The discovery signal generating means may be operable to encode said shared common cell identifier (or 'cluster identifier') into at least one of said pair of synchronisation signals and said identifier for said one cell into at least one further signal forming part of said discovery signal. 6 The discovery signal generating means may be operable to encode said shared common cell identifier (or 'cluster identifier') into said pair of synchronisation signals by encoding a cell group identifier into a first of said pair of synchronisation signals and a physical layer cell identity (PCI), identifying the cell within a cell group represented by the cell group identifier, into a second of said pair of synchronisation signals. The identifier for said one cell, said cell group identifier, and said PCI together may provide a global identifier for said one cell. The discovery signal generating means may be operable to encode said identifier for said one cell, as a global identifier for said one cell that does not depend on the common cell identifier, into said discovery signal. Communication apparatus for communicating with a mobile communication device in a communication system which uses a plurality of radio frames wherein each radio frame is subdivided in the time domain into a plurality of subframes, said communication apparatus comprising: means for operating a communication cell; means for generating discovery signals for use in a cell search procedure, each discovery signal comprising at least one synchronisation signal; configuring means for configuring a periodicity at which said communication apparatus should transmit said discovery signals wherein said configuring means is operable, when configuring said periodicity, to select said periodicity from any of a plurality of possible periodicities; and means for transmitting said discovery signals at said configured periodicity. The configuring means may be operable to configure a periodicity comprising a plurality of radio frames. The configuring means may be operable to configure a periodicity comprising a plurality of subframes. The configuring means may be operable, when configuring said periodicity, to select said periodicity from a range of possible periodicities. The range of possible periodicities may extend beyond 50 radio frames (or 50 subframes). According to one aspect of the invention there is provided a mobile communication device for communicating with communication apparatus in a cellular communication system using a plurality of radio frames wherein each radio frame is subdivided in the time domain into a plurality of subframes, each subframe is subdivided in the time domain into a plurality of slots, and each slot is subdivided in the time domain into a plurality of symbols, said mobile communication device comprising: means for performing a cell search procedure to discover a communication cell controlled by said communication apparatus; means for receiving a 7 discovery signal as part of said cell search procedure, wherein said discovery signal comprises a pair of synchronisation signals and at least one further signal, and wherein a symbol in which said at least one further signal is transmitted is separated, in the time domain, by no more than half a radio frame from at least one of said pair of synchronisation signals.; and means for identifying, and synchronising with, said cell based on said pair of synchronisation signals and said at least one further signal in said discovery signal. The mobile communication device may comprise means for performing frequency estimation based on said pair of synchronisation signals and at least one further signal in said discovery signal. According to another aspect of the invention a mobile communication device for communicating with communication apparatus in a cellular communication system using a plurality of radio frames wherein each radio frame is subdivided in the time domain into a plurality of subframes, each subframe is subdivided in the time domain into a plurality of slots, and each slot is subdivided in the time domain into a plurality of symbols, said mobile communication device comprising: means for performing a cell search procedure to discover a communication cell, controlled by said communication apparatus, as one cell of a cluster of cells each cell of which shares a common cell group identifier, and a common physical cell identity (PCI), which cell group identifier and common PCI together form a shared common cell identifier (or 'cluster identifier'); means for receiving a discovery signal as part of said cell search procedure, wherein said discovery signal comprises at least a pair of synchronisation signals and encoding an identifier for said one cell, wherein said identifier for said one cell is unique at least within said cluster of cells; and means for identifying said cell by extracting said identifier for said one cell from said discovery signal and for synchronising with said one cell based on said pair of synchronisation signals. According to another aspect of the invention there is provided a communication system comprising at least one communication apparatus and at least one mobile communication device each according to one of the associated aspects disclosed herein. According to another aspect of the invention there is provided a computer program product which may comprise instructions operable to program a programmable processor to implement communication apparatus or a communication device. According to another aspect of the invention there is provided a method performed by communication apparatus for communicating in a communication system which uses a plurality of radio frames wherein each radio frame is subdivided in the time domain into a plurality of subframes, each subframe is subdivided in the time domain into a plurality of slots, and each 8 slot is subdivided in the time domain into a plurality of symbols, said method comprising: operating a communication cell; generating discovery signals, for use in a cell search procedure, each discovery signal comprising a pair of synchronisation signals and at least one further signal; and transmitting each of said pair of synchronisation signals and said at least one further signal in a respective symbol of a radio frame; wherein said symbol in which said at least one further signal is transmitted separated, in the time domain, by no more than half a radio frame from at least one of said pair of synchronisation signals. According to another aspect of the invention there is provided a method performed by communication apparatus for communicating in a communication system which uses a plurality of radio frames wherein each radio frame is subdivided in the time domain into a plurality of subframes, each subframe is subdivided in the time domain into a plurality of slots, and each slot is subdivided in the time domain into a plurality of symbols, said method comprising: operating a communication cell as one cell of a cluster of cells each cell of which shares a common cell group identifier, and a common physical cell identity (PCI), which cell group identifier and common PCI together form a shared common cell identifier (or 'cluster identifier'); generating discovery signals, for use in a cell search procedure, each discovery signal comprising at least a pair of synchronisation signals and encoding an identifier for said one cell, wherein said identifier for said one cell is unique at least within said cluster of cells; and transmitting each of said pair of synchronisation signals in a respective symbol of a radio frame. According to another aspect of the invention there is provided a method performed by communication apparatus for communicating in a communication system which uses a plurality of radio frames wherein each radio frame is subdivided in the time domain into a plurality of subframes, said method comprising: operating a communication cell; generating discovery signals for use in a cell search procedure, each discovery signal comprising at least one synchronisation signal; configuring a periodicity at which said communication apparatus should transmit said discovery signals wherein, when configuring said periodicity, said periodicity is selected from a plurality of possible periodicities; and transmitting said discovery signals at said configured periodicity. According to another aspect of the invention there is provided a method performed by a mobile communication device for communicating with communication apparatus in a communication system which uses a plurality of radio frames wherein each radio frame is subdivided in the time domain into a plurality of subframes, each subframe is subdivided in the time domain into a plurality of slots, and each slot is subdivided in the time domain into a plurality of symbols, said method comprising: performing a cell search procedure to discover a 9 communication cell controlled by said communication apparatus; receiving a discovery signal as part of said cell search procedure, wherein said discovery signal comprises a pair of synchronisation signals and at least one further signal, and wherein a symbol in which said at least one further signal is transmitted is separated, in the time domain, by no more than half a radio frame from at least one of said pair of synchronisation signals.; and identifying, and synchronising with, said cell based on said pair of synchronisation signals and at least one further signal in said discovery signal. According to another aspect of the invention there is provided a method performed by a mobile communication device for communicating with communication apparatus in a communication system which uses a plurality of radio frames wherein each radio frame is subdivided in the time domain into a plurality of subframes, each subframe is subdivided in the time domain into a plurality of slots, and each slot is subdivided in the time domain into a plurality of symbols, said method comprising: performing a cell search procedure to discover a communication cell, controlled by said communication apparatus, as one cell of a cluster of cells each cell of which shares a common cell group identifier, and a common physical cell identity (PCI), which cell group identifier and common PCI together form a shared common cell identifier (or 'cluster identifier'); receiving a discovery signal as part of said cell search procedure, wherein said discovery signal comprises at least a pair of synchronisation signals and encoding an identifier for said one cell, wherein said identifier for said one cell is unique at least within said cluster of cells; and identifying said cell by extracting said identifier for said one cell from said discovery signal and for synchronising with said one cell based on said pair of synchronisation signals. Aspects of the invention extend to computer program products such as computer readable storage media having instructions stored thereon which are operable to program a programmable processor to carry out a method as described in the aspects and possibilities set out above or recited in the claims and/or to program a suitably adapted computer to provide the apparatus recited in any of the claims. Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently (or in combination with) any other disclosed and/or illustrated features. In particular but without limitation the features of any of the claims dependent from a particular independent claim may be introduced into that independent claim in any combination or individually. Brief Description of Drawings 10 Embodiments of the invention will now be described by way of example only with reference to the attached figures in which: Figure 1 schematically illustrates a telecommunication system; Figure 2 shows a simplified block diagram of a base station for the telecommunication 5 system of Figure 1; Figure 3 shows a simplified block diagram of a mobile communication device for the telecommunication system of Figure 1; Figure 4 shows an illustration of a typical radio frame used for communication in the telecommunication system of Figure 1; 10 Figure 5 shows an illustration of part of a typical subframe of the radio frame used for communication in the telecommunication system of Figure 1; Figure 6 shows an illustration of an example of how synchronisation signals may typically be transmitted in a macro cell of the telecommunication system of Figure 1; Figure 7 shows an illustration of an example of how discovery signals may be transmitted 15 in radio frames of a small cell of the telecommunication system of Figure 1; Figure 8 shows an illustration of an example of how discovery signals may be transmitted in a small cell of the telecommunication system of Figure 1; Figure 9 shows an illustration of another example of how discovery signals may be transmitted in a small cell of the telecommunication system of Figure 1; 20 Figure 10 shows an illustration of another example of how discovery signals may be transmitted in a small cell of the telecommunication system of Figure 1; Figure 11 shows an illustration of another example of how discovery signals may be transmitted in a small cell of the telecommunication system of Figure 1; Figure 12 shows an illustration of another example of how discovery signals may be 25 transmitted in a small cell of the telecommunication system of Figure 1; Figure 13 shows an illustration of another example of how discovery signals may be transmitted in a small cell of the telecommunication system of Figure 1; Figure 14 shows an illustration of another example of how discovery signals may be transmitted in a small cell of the telecommunication system of Figure 1; 30 Figure 15 shows an illustration of another example of how discovery signals may be transmitted in a small cell of the telecommunication system of Figure 1; Figure 16 shows a flow chart illustrating, in simplified form, one method by which a small cell base station of Figure 1 may generate and transmit a discovery signal for discovery by the mobile communication device in a cell search procedure for cell discovery purposes; 11 Figure 17 shows a flow chart illustrating, in simplified form, one method by which a mobile communication device of Figure 1 may use the discovery signal in a cell search procedure for cell discovery purposes; Figure 18 shows a flow chart illustrating, in simplified form, another method by which a 5 small cell base station of Figure 1 may generate and transmit a discovery signal for discovery by the mobile communication device in a cell search procedure for cell discovery purposes; and Figure 19 shows a flow chart illustrating, in simplified form, another method by which a mobile communication device of Figure 1 may use the discovery signal in a cell search procedure for cell discovery purposes. 10 Mode for Carrying out The invention Overview Figure 1 schematically illustrates a mobile (cellular) telecommunication system 1 in which a user of any of a plurality of mobile communication devices 3-1, 3-2, 3-3, 3-4, can 15 communicate with other users via one or more of a plurality of base stations 5, 6-1, 6-2, 6-3, 6-4. In the system illustrated in Figure 1, each base station 5, 6 shown is an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) base station capable of operating in a multi-carrier environment. In Figure 1, the base station labelled 5 comprises a so called ‘macro’ base station 20 operating a relatively large ‘macro’ cell 7. The other base stations 6 shown in Figure 1, each comprises a so called small cell base station (possibly a so called ‘pico’ base station, ‘Remote Radio Head (RRH)’, or other similar device capable of operating a small cell) operating a respective small cell 9-1, 9-2, 9-3, 9-4. The power used to provide small cells 9 is low relative to the power used for the macro 25 cell 7 and the small cells 9 are therefore small relative to the macro cell 7. As shown in Figure 1, in this example the geographical coverage of each of the small cells 9 falls completely within the geographical coverage of the macro cell 7 although partially overlapping geographical coverage with the macro cell 7 is, of course, possible. The macro base station 5 is configured to provide discovery signals in each radio frame 30 to allow a mobile communication device 3 to discover, and achieve synchronisation with, the macro base station 5. Specifically, the macro base station 5 transmits a discovery signal comprising a primary synchronisation signal (PSS) in a primary synchronisation channel (P-SCH) and a secondary synchronisation signal (SSS) in a secondary synchronisation channel (P-SCH), at predetermined fixed locations (in both frequency and time) within each radio frame, 12 and hence at a fixed periodicity, as set out in 3GPP TS 36.211 V10.2.0. In this example, the macro base station 5 is also configured to control operation of a ‘cluster’ comprising two small cell base stations 6-3 and 6-4 and hence their corresponding small cells 9-3, 9-4, that share a common physical cell identity (PCI), and group cell identity (and hence a common Cell ID 5 which is defined by the PCI and group cell identity), although the macro base station 5, and each small cell base station 6-3, 6-4 of the cluster represent a different distinct transmission point. Each small cell base station 6 is also configured to provide discovery signals on a physical discovery channel (PDCH) to allow a mobile communication device 3 to discover, and achieve synchronisation with, the small cell base station 6. However, unlike the discovery 10 signals transmitted by the macro base station 6, the discovery signals transmitted by each small base station 6 are not transmitted at a fixed periodicity but are, instead, transmitted at a flexible periodicity that can be configured, and reconfigured, by the small cell base station or the macro cell base station controlling the small cell base station 6. This extra flexibility is particularly beneficial because it allows a very wide range of periodicities (or ‘repetition intervals’) of 15 anything from a few subframes to several hundred radio frames or greater to be configured thereby helping to ensure that small cell base stations can be configured to consume less power and the overhead introduced by discovery signals can be reduced in dependence on the configured periodicity. Further, even in the presence of a relatively dense population of small cell base stations 6 the chances of several respective PSS and SSS being transmitted at the same 20 time (or within close temporal proximity to one another) for different small cell base stations, is relatively low. The discovery signals provided by each small cell base station 6 beneficially re-use the structure of the PSS and SSS used by the macro base station 5 thereby providing at least some backwards compatibility with legacy mobile communication devices. In the exemplary 25 embodiments described herein, however, the discovery signals provided by each small cell base station 6 are, however, further enhanced: to reduce the size of the search window that the mobile communication device 3 needs to employ; to improve the speed with which a search can be carried out; to improve the reliability of a cell search procedure; to enhance time and/or frequency tracking; to allow measurement (e.g. of Reference Signal Received Quality (RSRQ) / 30 Reference Signal Received Power (RSRP) and/or to allow additional information (such as additional cell identification / transmission point identification information) to be represented by the discovery signal and obtained, at the time of discovery, by suitably enhanced mobile communication devices 3. In some exemplary embodiments, the discovery signals are enhanced in the time domain 13 by including additional PSS and/or SSS type signals in each subframe. In other exemplary embodiments, the discovery signals are enhanced in the frequency domain by expanding the frequency range over which the PSS and/or SSS type signals are transmitted. Moreover, it will be appreciated that by combining the time-domain and frequency domain discovery signal 5 enhancements of different exemplary embodiments the respective benefits of the different enhancements combine together to provide a greater cumulative benefit albeit at the expense of greater discovery signal overhead. The introduction of the new discovery signal (on the PDCH) therefore helps to facilitate the search for, acquisition of, and synchronisation with a small cell base station, by a mobile 10 communication device 3. Depending on the way in which the new discovery signal is implemented, one or more of the following additional benefits arise compared, for example, with using a conventional P-SCH / S-SCH structure: power savings in mobile communication device and/or small cell base station; a relatively quick search for/discovery of a small cell; relatively rapid acquisition of measurement results for the purposes of handover, thereby facilitating rapid 15 handover operations; and/or the support of a greater number of physical Cell IDs and/or dedicated transmission point identities. Base Station Figure 2 is a block diagram illustrating the main components of a base station 5, 6 as shown in Figure 1. The base station 5, 6 comprises an E-UTRAN multi-carrier capable base 20 station comprising a transceiver circuit 31 which is operable to transmit signals to, and to receive signals from, the mobile communication devices 3 via at least one antenna 33. The base station 5, 6 is also operable to transmit signals to and to receive signals from: a core network via a network interface 35 (optionally via a small cell gateway or the like in the case of a small cell base station (not shown)); and other base stations 5, 6 in the vicinity via a base station (or so 25 called ‘X2’) interface 36. The operation of the transceiver circuit 31 is controlled by a controller 37 in accordance with software stored in memory 39. The software includes, among other things, an operating system 41, a communication control module 42, a synchronisation module 47, and a cell / transmission point identification module 49. 30 The communication control module 42 is operable to control communication with the mobile communication devices 3 via the antenna 33 and with the core network and other base stations via the network interface 35 and the X2 interface 36 respectively. The synchronisation module 47 manages the generation and transmission of discovery signals at appropriate locations in a radio frame 210 (see Fig. 4) as required and, in the case of the small cell base station 6, the 14 configuration and reconfiguration of the periodicity of the discovery signals, and the encoding of appropriate information into the discovery signal as appropriate. The cell / transmission point identification module 49 generates identification information for the cell / transmission point such as the Cell ID (cell group ID / physical cell ID (PCI)) and/or transmission point ID (e.g. for 5 small cells sharing the same Cell ID as other cells in a cluster of cells) for encoding into the discovery signals by the synchronisation module 47. In the above description, the base station 5 is described for ease of understanding as having a number of discrete modules. Whilst these modules may be provided in this way for certain applications, for example where an existing system has been modified to implement the 10 invention, in other applications, for example in systems designed with the inventive features in mind from the outset, these modules may be built into the overall operating system or code and so these modules may not be discernible as discrete entities. Mobile Communication Device Figure 3 is a block diagram illustrating the main components of the mobile 15 communication devices 3 shown in Figure 1. Each mobile communication device 3 comprises a mobile (or ‘cell’ telephone) capable of operating in a multi-carrier environment. The mobile communication device 3 comprises a transceiver circuit 51 which is operable to transmit signals to, and to receive signals from, the base stations 5, 6 via at least one antenna 53. The operation of the transceiver circuit 51 is controlled by a controller 57 in accordance with software stored in 20 memory 59. The software includes, among other things, an operating system 51, a communication control module 62, a synchronisation module 67, a cell / transmission point identification module 69, a timing / frequency estimation module 71 and a measurement module 73. The communication control module 62 is operable for managing communication with the 25 base stations 5, 6. The synchronisation module 67 manages synchronisation of the mobile communication device 3 with the radio frame / subframe timing of the base station 5, 6. The synchronisation module 67 also manages the identification of the discovery signal configuration such as the location of the PSS/SSS within each radio frame 210 (see Fig. 4) and the decoding of appropriate information from the discovery signal as appropriate. The cell / transmission point 30 identification module 69 determines identification information for the cell / transmission point such as the Cell ID (cell group ID / PCI) and/or transmission point ID (e.g. for small cells sharing the same Cell ID as other cells in a cluster of cells) from the information encoded into the discovery signals by the synchronisation module 67. The timing / frequency estimation module 71 is configured for performing timing and/or 15 frequency estimation using the discovery signals generated and transmitted by the small cell base station 6. The measurement module 73 is configured for performing measurements (e.g. RSRP / RSRQ measurements) on the discovery signals generated and transmitted by the small cell base station 6. 5 In the above description, the mobile communication device 3 is described for ease of understanding as having a number of discrete modules. Whilst these modules may be provided in this way for certain applications, for example where an existing system has been modified to implement the invention, in other applications, for example in systems designed with the inventive features in mind from the outset, these modules may be built into the overall operating 10 system or code and so these modules may not be discernible as discrete entities. Discovery Signal Configuration The way in which the discovery signals are configured in the present exemplary embodiment will now be described in more detail, by way of example only, with reference to Figures 4 to 15. 15 Figure 4 illustrates the structure of a radio frame 210 of the type used for communication by the base stations 5, 6 and mobile communication devices 3 of the mobile (cellular) telecommunication system 1. As seen in Figure 4, each base station 5 is configured to transmit control information and data to associated mobile communication devices using radio frames 210. Each radio frame 210, in this exemplary embodiment, is 10ms long and as seen in Figure 4 20 comprises a plurality of orthogonal frequency division multiplexing (OFDM) subframes 230 (in this exemplary embodiment ten 1ms subframes which are indexed ‘0’ through ‘9’ make up a radio frame 210). Each subframe comprises a pair of slots 232a and 232b (in this exemplary embodiment 0.5ms long). For the purposes of referencing, the slots 232 are typically referred to by index numbers ranging from ‘0’ to ‘19’ in chronological order (from left to right on Figure 4) 25 with the first slot 232a of each subframe 230 having an even number and the second slot 232b having an odd number. Part of the first (‘even numbered’) slot of each subframe 230 comprises a so called ‘control’ region 231 that is generally reserved for the transmission of control information. The remainder of the first (‘even numbered’) slot of each subframe 230 and the second (‘odd numbered’) slot of each subframe 230 comprises a so called ‘data’ region 233 that 30 is generally used for the transmission of data, for example in a Physical Downlink Shared Channel (PDSCH). Figure 5 shows a resource grid for part of one of the OFDM subframes 230 of Figure 4. The resource grid shown is for a resource block (RB) pair 310a, 310b (represented by hatched region) with each RB 310a, 310b of the pair having, for example, a resource grid similar to that 16 described in section 6.2 of the 3rd Generation Partnership Project (3GPP) Technical Standard (TS) 36.211 V10.2.0 and shown in Figure 6.2.2-1 of that standard. As seen in Figure 5, each resource block 310a and 310b is part of a respective slot 232a and 232b of the subframe 230. Each resource block 310a, 310b comprises a set of resource 5 elements 335 defined in frequency by 12 subcarrier frequencies (rows) and in time by 7 symbols (columns). In this embodiment, the control region 231 comprises the resource elements 335 of the first three OFDM symbols of the first slot 232a of each subframe 230. The remaining resource elements 335 of the first slot 232a and the resource elements 335 of the second slot 232b form the data region 333. 10 As described above, each base station 5, 6 is configured to provide discovery signals at particular locations (in frequency and time) in corresponding radio frames 210 to allow the mobile communication device 3 to discover and achieve synchronisation with the base station 5, 6. Each discovery signal comprises a PSS that can be used by the mobile communication 15 device 3 to synchronise receipt and transmission of each symbol, slot and subframe with the corresponding symbol, slot and/or subframe timings of the corresponding base station 5, 6. Further, the PSS is also used by the mobile communication device 3 to identify other information about the cell to which it relates, for example, cell identity information such as a physical layer cell identity (PCI). Each discovery signal also comprises an SSS that is used by the mobile 20 communication device 3 to synchronise receipt and transmission of each radio frame 210 with the associated frame timings of the corresponding base station 5, 6. The SSS is also used by the mobile communication device 3 to identify other information about the cell to which they relate, for example cell group identity information such as the physical layer cell identity group for the base station 5, 6. 25 Figure 6 illustrates how the discovery signals are configured in the radio frames 230 for the macro cell 7 of the macro base station 5. As seen in Figure 6, in the case of macro cell 7, the macro base station 5 always provides the synchronisation signals (PSS/SSS) in the last two symbols of the first (even numbered) slot 232a in the first subframe 230 (subframe #0) and in the last two symbols of the first (even 30 numbered) slot 232a in the sixth subframe 230 (subframe #5). The PSS is transmitted in the last symbol of the first slot 232a of the first and sixth subframes, whilst the SSS is transmitted in the second to last symbol of the first slot 232a of the first and sixth subframes 230 (i.e. the last symbol of slot #0 and slot #10). The PSS and SSS are each allocated the central 62 subcarriers belonging to symbols in 17 which they are respectively located. The 5 resource elements above and below the synchronisation signals are not used for transmission and hence the PSS and SSS each account for the central 72 subcarriers (6 resource blocks). Thus, the PSS and SSS transmissions each use six resource blocks (with 10 resource elements left unused) and the indexes of the resource 5 blocks (resource block numbers) within the frequency band covered by the macro cell 7 are always the same. The sequence d(n) used for the PSS is generated from a frequency-domain Zadoff-Chu sequence as follows: _ jm un(n+1) du ( n )H «(n+1)( n+2) J e 63 n = 0,1,...30 1)( e 63 n = 31,32,...,61 10 where the Zadoff-Chu root sequence index u depends on the PCI and is one of the indices 25, 29 and 34. The sequence d(0),...,d(61) used for the SSS comprises an interleaved concatenation of two length-31 binary sequences. The concatenated sequence is scrambled with a scrambling sequence given by the PSS. 15 The combination of two length-31 sequences defining the secondary synchronization signal differs between subframe #0 and subframe #5 according to: (m0) (n)c0(n) in subframe 0 0 1 \s0 d(2n)= d(2n + 1) = ( m 1)(n)c0(n) in subframe 5 m 1)(n)c 1(n)z1(m0)(n) in subframe 0 (m (n)c1{n)z1( m 1){n) in subframe 5 where 0