Antenna System for Shared Operation This invention relates an antenna system for shared operation by multiple operators, such as for example (but without limitation) a phased array antenna system having a plurality of antennas, or a single antenna assembly having a plurality of antenna stacks within a radome. The antenna system of the invention is relevant to many telecommunications systems, such as for example cellular mobile radio networks commonly referred to as mobile telephone networks, It is relevant to antenna sharing for first generation (1G) mobile telephone networks such as TAGS, second generation (2G) mobile telephone networks such as GSM and D-AMPS (IS136), third generation (3G) mobile telephone networks such as the Universal Mobile Telephone System (UMTS), and other cellular radio networks. Operators of conventional cellular radio networks generally employ their own base stations each of which is connected to one or more antennas. However, the numbers of cellular radio networks and operators are increasing world-wide: this has led to increases in the number of antenna sites and the number of antennas per site. In consequence, this in turn is leading to planning or zoning restrictions imposed by legal authorities, which are increasingly placing limits on numbers of antenna sites, and on obtrusiveness of antenna structures, in order to minimise visual impact on the environment. Antenna sharing provides a solution to the problem of limiting site and antenna numbers. However, it introduces the problem of increased complexity and bulk of apparatus for signal transfer between base stations and antenna system, and consequent increase in weight supported by, wind loading experienced by and strength required of an antenna support mast. It can also require multiple operators to have a common angle of electrical tilt of an antenna, contrary to a general requirement for each operator to have an individual angle of electrical tilt. An antenna system for shared operation by multiple operators or base stations is disclosed by European patent no. EP 0 566 603 (inventor E Prokkola). This patent describes multiple base stations of different types (GSM, ETACS, TAGS) connected to respective band-pass transmit filters and thence to a common transmit antenna. The problems of increased complexity, common angle electrical tilt and bulk of signal transfer apparatus are not addressed. As common general knowledge in the field of cellular radio networks, and as will be described later in more detail, it is known to provide a shared antenna system with a respective antenna stack for each co-located base station per sector of antenna coverage. In the case of such a system having transmit and receive diversity (as described later), two signal feeders per sector per base station are required to connect base stations to antenna stacks, each feeder carrying both transmit and receive signals. For three base stations and three sectors, eighteen feeders are required, and thirty-six for six sectors. Despite the large number of feeders, this design does not make it possible to use separate transmit and receive feeders for signals as may be required to attain a low level of interference from intermodulation products, which arise from mixing of different signal frequencies. An antenna system of this general .type is disclosed by published international application no. WO 03/043127, but it shows additional feeders being used to separate transmit and receive signals. In cellular radio networks, a feeder is a coaxial cable weighing typically 1.26kg/metre and is used to carry signals at a frequency typically in the range 400 MHz to 2 GHz. An antenna mast may be required to support the weight of each feeder, typically 30 metres in length (37.8 kg) - or even up to 60 metres (75.6 kg). Moreover, each feeder increases wind resistance and therefore wind loading on an antenna mast. In consequence, the number of feeders is an important consideration in mast design. s In order to improve transmission performance, it is known to use diversity, i.e. to receive and/or transmit two or more diverse signals. The diverse signals are processed either individually or in combination. There are three common types of diversity in transmit mode, a) frequency diversity, b) spatial diversity and c) polarisation diversity. In transmit mode, a mobile cellular radio handset has a single transmitting antenna which transmits a carrier wave with a single polarisation. A base station uses a dual polarisation antenna with one antenna element (or set of elements) having a +45 degree polarisation and the other -45 degree polarisation, One signal from the handset therefore gives rise to two signals at the antenna. The base station processes both receive signals in order to obtain the best signal. This approach combats changes in the polarisation of radio signals due to different orientations of the mobile antenna and reflection at buildings etc., which cause signals to be received at a base station antenna with multiple polarisations. i In polarisation diversity used for transmit mode, a base station transmits one signal on a +45 degrees antenna element and another signal on a -45 degrees antenna element. The two signals have the same carrier frequency but different modulations. A mobile cellular radio handset receives the two signals on a single antenna, but since they are modulated differently a handset can process them to obtain improved reception. Cellular radio systems have used base station receive diversity for some time, and transmit diversity is now being considered as a means of further improving transmission performance. The present invention provides an antenna system for shared operation having a plurality of antennas for use with multiple base stations, characterised in that each antenna is arranged to transmit and receive for at least one respective base station, the antenna system is arranged to combine on to a single feeder signals with different non-contiguous frequencies associated with a plurality of the base stations, and to separate on to different feeders signals with contiguous or overlapping or like frequencies for at least one of transfer of signals from antennas to base stations in receive mode and transfer of signals from base stations to antennas in transmit mode. Here an antenna may be a single phased array antenna, or antenna stack, mounted in a single radome, or multi-phased array antennas, or antenna stacks, mounted either in a single radome, or multiple radomes. The invention provides the advantage that it reduces the number of feeders required compared to comparable prior art, while preserving capability for having different angles of electrical tilt at different antennas should this be required. Moreover, it may be configured to provide one antenna per operator, in which case an individual angle of electrical tilt is obtainable for each operator. The antenna system may be arranged to combine on to a single feeder transmit and receive signals associated with like polarisation and with a plurality of base stations, and to separate on to different feeders transmit and receive signals associated with different polarisations. It may have: a) one bandpass filter per base station per polarisation for combining base station transmit signals on to feeders; b) one bandpass filter per antenna per polarisation for separating base station transmit signals for routing to different antennas; and c) two bandpass filters and a low noise amplifier per antenna per polarisation for combining antenna receive signals on to feeders. The antenna system may be arranged to combine transmit signals on to different feeders to receive signals. It may have: a) one bandpass filter per base station per polarisation for combining base station transmit signals on to feeders; b) one bandpass filter per antenna per polarisation for separating base station transmit signals for routing .to different antennas; c) one bandpass filter and one low noise amplifier per antenna per polarisation and one combiner per polarisation for combining antenna receive signals on to feeders; and d) one splitter per polarisation for routing antenna receive signals from feeders to different base stations. The antenna system may be arranged to: a) combine base station transmit signals associated with like polarisation onand one low noise amplifier (e.g. 186a) per antenna per polarisation and one combiner (107, 115) per polarisation for combining antenna receive signals on to feeders (109,117); and d) one splitter (111, 119) per polarisation for routing antenna receive signals from feeders (109,117) to different base stations (AS101 to AS103). 23. A method according to Claim 16 characterised in that it includes: a) combining base station transmit signals (e.g. TX(+)) associated with like polarisation on to a single feeder (e.g. 162); b) combining antenna receive signals (e.g. RX(+)) associated with like polarisation on to a single feeder (e.g. 1.09); c) separating base station transmit signals (TX(+), TX(-)) associated with different polarisations on to different feeders (162,164); and d) separating antenna receive signals (RX(+), RX(-)) associated with different polarisations on to different feeders (109,117). 24. A method according to Claim 16 implementing receive diversity but not transmit diversity, characterised in that the method includes combining on to a single feeder (262) transmit and receive signals (TX(+)/RX(+)) associated with one polarisation and separating on to another feeder (217) receive signals (RX(-)) associated with a different polarisation. 25. A method according to Claim 24 characterised in that it is implemented using: a) one bandpass filter (e.g. 258a) per base station (251 to 253) for combining base station transmit signals on to a first feeder (262); b) one bandpass filter (e.g. 270a) per antenna (e.g. AS201) for separating base station transmit signals for routing to different antennas (AS201 to AS203); c) two bandpass filters (e.g. 284a, 288a) and a low noise amplifier (e.g. 286a) per antenna (e.g. AS201) for combining antenna receive signals of one polarisation on to a feeder (262), which is also arranged to carry transmit signals associated with like polarisation; and d) one bandpass filter (e.g. 268a) and one low noise amplifier (e.g. 269a). per antenna and one combiner (215) for combining antenna receive signals associated with another polarisation on to a feeder (217), and one splitter (219) forrouting such receive signals from the feeder (217) to different base stations (251 to 253). 26. A method according to Claim 16 arranged for receive diversity but not transmit diversity, characterised in that it includes separating on to different feeders (362, 309, 317) transmit and receive signals (TX(+), RX(+), RX(-)) and also receive signals (RX(+), RX(-)) associated with different polarisations. 27. A method according to Claim 26 characterised in that it is implemented using: a) one bandpass filter (e.g. 358a) per base station (351 to 353) for combining base station transmit signals on to a first feeder (362); b) one bandpass filter (e.g. 370a) per antenna (e.g. AS301) for separating base station transmit signals for routing to different antennas (AS301 to AS303); c) a bandpass filter (e.g. 384a) and a low noise amplifier (e.g. 386a) per , antenna (e.g. A S201) for c ombining a ntenna receive s ignals a ssociated with one polarisation on to a second feeder (309); and d) one bandpass filter (e.g. 368a) and one low noise amplifier (e.g. 369a) per antenna and one combiner (315) for combining antenna receive signals of another polarisation on to a third feeder (317), and one splitter (319) for routing such receive signals from the third feeder (317) to. different base stations (351 to 353). 28. A method according to Claim 16 characterised in that each base station has a respective transmit/receive band (e.g. band 1) spaced from anther such band by an intervening gap (e.g. IBG1), and the method includes arranging frequencies of pairs of transmit and receive signals for duplex filtering with intervening frequency differences which are equal to at least the combined width of a gap (e.g. IBG1) . and a band (e.g. band 1). 29. A method .of shared operation of an antenna system (50) incorporating an antenna assembly (AA) having a plurality of antennas (AS1 to ASS) arranged to implement diversity and for use with multiple base stations (51 to 53), characterised in that the method includes combining on to a single feeder (62, 64) signals associated both with like diversity type and with a plurality of base stations (51 to 53), and to separate on to different feeders (62,64) signals associated with different diversity types for at least one of transfer of signals from antennas (AS1 to ASS) to base stations (51 to 53) in receive mode and transfer of signals from base stations (51 to 53) to antennas (AS1 to ASS) in transmit mode. 30. A method of shared operation of antenna system (50) with multiple base stations (51 to 53), the antenna system (50) incorporating an antenna assembly (M) having a plurality of antennas (AS1 to ASS) each associated with a plurality of polarisations, characterised in that the method includes combining on to a single feeder (62, 64) signals associated both with like polarisation and with a plurality of • the base stations (51 to 53), and separating on to different feeders (62, 64) signals associated with different polarisations for at least one of transfer of signals from antennas (AS1 to A S3) to base stations (51 to 53.) in receive mode and transfer of signals from base stations (51 to 53) to antennas (AS1 to ASS) in transmit mode.