MICROWAVE PLASMA REACTORS AND SUBSTRATES FOR SYNTHETIC DIAMOND MANUFACTURE Field of Invention The present invention relates to a microwave plasma reactor for manufacturing synthetic diamond material using chemical vapour deposition techniques. Certain embodiments relate to substrates for use in a microwave plasma reactor for synthetic diamond manufacture. Background of Invention Chemical vapour deposition (CVD) processes for synthesis of diamond material are now well known in the art. Useful background information relating to the chemical vapour deposition of diamond materials may be found in a special issue of the Journal of Physics: Condensed Matter, Vol. 21, No. 36 (2009) which is dedicated to diamond related technology. For example, the review article by R.S Balmer et al. gives a comprehensive overview of CVD diamond materials, technology and applications (see "Chemical vapour deposition synthetic diamond: materials, technology and applications" J. Phys. : Condensed Matter, Vol. 21, No. 36 (2009) 364221). Being in the region where diamond is metastable compared to graphite, synthesis of diamond under CVD conditions is driven by surface kinetics and not bulk thermodynamics. Diamond synthesis by CVD is normally performed using a small fraction of carbon (typically <5%), typically in the form of methane although other carbon containing gases may be utilized, in an excess of molecular hydrogen. If molecular hydrogen is heated to temperatures in excess of 2000 K, there is a significant dissociation to atomic hydrogen. In the presence of a suitable substrate material, diamond can be deposited. Atomic hydrogen is essential to the process because it selectively etches off non-diamond carbon from the substrate such that diamond growth can occur. Various methods are available for heating carbon containing gas species and molecular hydrogen in order to generate the reactive carbon containing radicals and atomic hydrogen required for CVD diamond growth including arc-jet, hot filament, DC arc, oxy-acetylene flame, and microwave plasma. Methods that involve electrodes, such as DC arc plasmas, can have disadvantages due to electrode erosion and incorporation of material into the diamond. Combustion methods avoid the electrode erosion problem but are reliant on relatively expensive feed gases that must be purified to levels consistent with high quality diamond growth. Also the temperature of the flame, even when combusting oxy-acetylene mixes, is insufficient to achieve a substantial fraction of atomic hydrogen in the gas stream and the methods rely on concentrating the flux of gas in a localized area to achieve reasonable growth rates. Perhaps the principal reason why combustion is not widely used for bulk diamond growth is the cost in terms of kWh of energy that can be extracted. Compared to electricity, high purity acetylene and oxygen are an expensive way to generate heat. Hot filament reactors while appearing superficially simple have the disadvantage of being restricted to use at lower gas pressures which are required to ensure relatively effective transport of their limited quantities of atomic hydrogen to a growth surface. In light of the above, it has been found that microwave plasma is the most effective method for driving CVD diamond deposition in terms of the combination of power efficiency, growth rate, growth area, and purity of product which is obtainable. A microwave plasma activated CVD diamond synthesis system typically comprises a plasma reactor vessel coupled both to a supply of source gases and to a microwave power source. The plasma reactor vessel is configured to form a resonance cavity supporting a standing microwave. Source gases including a carbon source and molecular hydrogen are fed into the plasma reactor vessel and can be activated by the standing microwave to form a plasma in high field regions. If a suitable substrate is provided in close proximity to the plasma, reactive carbon containing radicals can diffuse from the plasma to the substrate and be deposited thereon. Atomic hydrogen can also diffuse from the plasma to the substrate and selectively etch off non-diamond carbon from the substrate such that diamond growth can occur. A range of possible microwave plasma reactors for diamond film growth via a chemical vapour deposition (CVD) process are known in the art. Such reactors have a variety of different designs. Common features include: a plasma chamber; a substrate holder disposed in the plasma chamber; a microwave generator for forming the plasma; a coupling configuration for feeding microwaves from the microwave generator into the plasma chamber; a gas flow system for feeding process gases into the plasma chamber and removing them therefrom; and a temperature control system for controlling the temperature of a substrate on the substrate holder. The present inventors consider that when designing a microwave plasma reactor process for diamond film growth, to achieve a successful industrial process requires the assessment of a number of considerations including: chamber and microwave power coupling configuration; gas flow characteristics; and substrate design and temperature control. Certain embodiments of the present invention are primarily concerned with the aspects of substrate design and temperature control. The most commonly used substrate for CVD diamond growth is silicon. One problem with using silicon as a substrate for CVD diamond growth in a microwave plasma growth process is power absorption by the silicon at high temperatures, leading to thermal runaway and fracture. Another problem is that silicon is readily incorporated into CVD diamond during growth, being particularly visible as the 737nm Si-V defect. As such, the use of a silicon substrate can detrimentally affect the purity of the CVD diamond product. Yet another problem is that after growth of a CVD diamond wafer on a silicon substrate, recovery of the CVD diamond wafer may require, for example, one of mechanical or acid removal. These additional processing steps increase the time and expense of an industrially implemented process. In light of the above, it is evident that it would be desirable to find an alternative substrate material which solves these problems. One possibility for a substrate material is a carbide forming refractory metal such as tungsten, molybdenum, niobium, or alloys thereof. Such substrates have already been proposed in the art. For example, US5261959 suggests a refractory metal substrate material such as molybdenum in the form of a planar circular disk. Alternatively, Whitfield et al. suggest the use of a tungsten substrate (see "Nucleation and growth of diamond films on single crystal and polycrystalline tungsten substrates", Diamond and Related Materials, Volume 9, Issues 3-6, April-May 2000, Pages 262-268). Specifically, Whitfield et al. disclose the use of a polycrystalline tungsten disc 6.3 mm thick and 50 mm in diameter and a single crystal tungsten disc 6.3 mm thick and 8 mm in diameter in a 2.45 GHz microwave plasma reactor. The substrates were subjected to preparation steps including polishing to a mirror finish with a 1-3 micrometer diamond abrasive and cleaning via ultrasonic washing and an in situ plasma etch. Substrate temperatures were monitored using optical pyrometry and an embedded thermocouple during CVD diamond growth. Spontaneous delamination of the CVD diamond wafer from the tungsten substrate on cooling after growth is also disclosed to yield a free-standing diamond wafer due to the differences in thermal expansion coefficient between the CVD diamond wafer and the tungsten substrate. Whitfield et al. note that generally in their experiments the substrates were not reused but in the few cases where re-use did occur, substrates were lapped and polished for at least 24 hours to remove the thin carbide layer formed during the previous growth run. In light of the above, it is evident that carbide forming refractory metals may provide an attractive alternative to silicon substrates. Despite this, the present inventors have experienced a number of problems when using such substrates. These include: nonuniform CVD diamond growth over the substrate; delamination of the CVD diamond wafer from the substrate during CVD diamond growth; and crack initiation and propagation during cooling after growth of the CVD diamond wafer. These problems tend to be exacerbated when larger substrates are used for growing large area polycrystalline diamond discs (e.g. 80 mm diameter or more) or when growing a plurality of single crystal diamonds in a single growth run on a plurality of single crystal diamond substrates adhered to a refractory metal substrate over a relatively large area (e.g. 80 mm diameter or more). This is particularly problematic as there is an on going need to increase the area over which high quality, uniform CVD diamond can be grown. Furthermore, these problems tend to be exacerbated when the substrates are reused in subsequent growth runs. This is particularly problematic as the substrates are expensive and reuse is desirable in an economically competitive industrial process. It is an aim of certain embodiments of the present invention to at least partially address one or more of these problems. In particular, it is an aim of certain embodiments of the present invention to provide more uniform and/or more consistent CVD diamond products. Summary of Invention According to a first aspect of the present invention there is provided a microwave plasma reactor for manufacturing synthetic diamond material via chemical vapour deposition, the microwave plasma reactor comprising: a microwave generator configured to generate microwaves at a frequency f; a plasma chamber comprising a base, a top plate, and a side wall extending from said base to said top plate defining a resonance cavity for supporting a microwave resonance mode between the base and the top plate; a microwave coupling configuration for feeding microwaves from the microwave generator into the plasma chamber; a gas flow system for feeding process gases into the plasma chamber and removing them therefrom; a substrate holder disposed in the plasma chamber and comprising a supporting surface for supporting a substrate; and a substrate disposed on the supporting surface, the substrate having a growth surface on which the synthetic diamond material is to be deposited in use, wherein the substrate dimensions and location within the resonance cavity are selected to generate a localized axisymmetric Ez electric field profile across the growth surface in use, the localized axisymmetric Ez electric field profile comprising a substantially flat central portion bound by a ring of higher electric field, the substantially flat central portion extending over at least 60% of an area of the growth surface of the substrate and having an Ez electric field variation of no more than ±10% of a central Ez electric field strength, the ring of higher electric field being disposed around the central portion and having a peak Ez electric field strength in a range 10% to 50% higher than the central Ez electric field strength. According to an alternative definition of the first aspect of the present invention there is provided a microwave plasma reactor for manufacturing synthetic diamond material via chemical vapour deposition, the microwave plasma reactor comprising: a microwave generator configured to generate microwaves at a frequency f; a plasma chamber comprising a base, a top plate, and a side wall extending from said base to said top plate defining a resonance cavity for supporting a microwave resonance mode between the base and the top plate; a microwave coupling configuration for feeding microwaves from the microwave generator into the plasma chamber; a gas flow system for feeding process gases into the plasma chamber and removing them therefrom; a substrate holder disposed in the plasma chamber and comprising a supporting surface for supporting a substrate; and a substrate disposed on the supporting surface, the substrate having a growth surface on which the synthetic diamond material is to be deposited in use, wherein a ratio of substrate diameter / height of the growth surface of the substrate is in a range 10 to 14, 1 1 to 13.5, or 1 1.0 to 12.5, wherein the height of the growth surface of the substrate is relative to a mean height of a surface surrounding the substrate. According to a second aspect of the present invention there is provided a substrate for use in a microwave plasma reactor according to the first aspect of the invention, the substrate comprising: a cylindrical disc of a carbide forming refractory metal having a flat growth surface on which CVD diamond is to be grown and a flat supporting surface opposed to said growth surface, wherein the cylindrical disc has a diameter of 80 mm or more, wherein the growth surface has a flatness variation no more than 100 μηι, and wherein the supporting surface has a flatness variation no more than 100 μπι. According to a third aspect of the present invention there is provided a method of manufacturing synthetic diamond material using a chemical vapour deposition process, the method comprising: providing a reactor configured for manufacturing synthetic diamond material; locating a substrate on a substrate holder within the reactor, the substrate comprising a growth surface on which synthetic diamond material is to be grown; feeding process gases into the reactor; and growing synthetic diamond material on the growth surface of the substrate, wherein the method further comprises: taking at least two temperature measurements, including one or more measurements in a central region of the growth surface of the substrate and one or more measurements in a peripheral region of the growth surface of the substrate during growth of the synthetic diamond material; and controlling a temperature difference between the central region and the peripheral region of the growth surface of the substrate during growth of the synthetic diamond material based on the at least two temperature measurements, wherein the temperature of the growth surface of the substrate during growth of the synthetic diamond material is controlled to fulfil the condition 5°C < Tc - Te < 120°C, where Tc is a temperature in the central region of the growth surface of the substrate and Te is a temperature in the peripheral region of the growth surface of the substrate. According to a fourth aspect of the present invention there is provided a method of manufacturing synthetic diamond material using a chemical vapour deposition process, the method comprising: providing a reactor configured for manufacturing synthetic diamond material; locating a substrate over a substrate holder within the reactor, the substrate comprising a growth surface on which synthetic diamond material is to be grown; feeding process gases into the reactor; and growing synthetic diamond material on the growth surface of the substrate, wherein the synthetic diamond material is grown to form a polycrystalline diamond wafer having a diameter of at least 120 mm, and wherein the polycrystalline diamond wafer is spontaneously delaminated from the substrate on cooling after the chemical vapour deposition process is completed to yield a free-standing polycrystalline diamond wafer which is substantially crack free over at least a central region thereof, wherein the central region is at least 70% of a total area of the free-standing polycrystalline diamond wafer, and wherein the central region has no cracks which intersect both external major faces of the free-standing poly crystalline diamond wafer and extend greater than 2 mm in length. Brief Description of the Drawings For a better understanding of the present invention and to show how the same may be carried into effect, embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figures 1(a) to 1(c) show electric field profile plots for varying heights of substrate within a microwave plasma reactor; Figures 2(a) to 2(c) show how the height of the growth surface of the substrate is calculated relative to relative to a mean height of a surface surrounding the substrate; Figure 3 shows a cross-sectional view of a microwave plasma reactor configured according to an embodiment of the present invention; Figure 4 shows a plan view of a portion of the microwave plasma reactor in more detail illustrating a substrate holder and spacer wires; Figure 5 shows a cross-sectional view of another microwave plasma reactor configured to include a temperature modifying ring disposed over the substrate holder and around the substrate; Figure 6 shows the temperature modifying ring in more detail; Figure 7 shows a substrate according to an embodiment of the present invention; and Figures 8(a) to 8(d) show various possible alternative substrate configurations. Detailed Description of Certain Embodiments It is considered desirable to provide a microwave plasma reactor which is configured to form a uniform, large area plasma above a substrate in order to provide uniform CVD diamond growth over a large area of the substrate. Intuitively, one would expect that a microwave plasma reactor should be configured to support a uniform electric field above the substrate in order to form such a uniform plasma. The first aspect of the present invention is based on the seemingly counter-intuitive finding that a non-uniform electric field of a particular form can produce a more uniform plasma over a larger area than a corresponding uniform underlying electric field and that this can lead to more uniform CVD diamond growth over larger areas. In particular, the present inventors have found that it is preferable to form an electric field having an axisymmetric Ez profile comprising a substantially flat central portion bound by a ring of higher electric field, the substantially flat central portion extending over at least 60% of an area of the growth surface of the substrate and having an Ez electric field variation of no more than ±10%, 8%, 6%, 5%, 4%, 3%, 2%, or 1% of a central Ez electric field strength. The ring of higher electric field is disposed around the central portion and has a peak Ez electric field strength in a range 10% to 50%, 10% to 40%, 15%) to 30%), or 15%) to 25% higher than the central Ez electric field strength. It has been found that the ring of higher electric field can aid in pulling the plasma outwards to form a flat, large area plasma above the substrate. Furthermore, as the plasma edges have higher radiative and convective losses, the ring of higher electric field is considered advantageous to compensate for such losses. The uniform plasma may then provide uniform heat flow towards the underlying substrate and uniform transport of active species to the growth surface of the substrate to yield uniform CVD diamond growth over large areas. It has been found that an electric field profile as previously described can be formed by selecting suitable substrate dimensions and positioning the substrate in the correct location within the resonance cavity of the plasma reactor. In this regard, it is possible to model the electric field for particular chamber configurations to determine the electric field profile above the substrate growth surface. The electric field profile may be modelled by performing electromagnetic field calculations for a resonance cavity of specified dimensions at resonance (not necessarily at driving frequency). The calculations can be made using an Eigenvalue differential equation solver. The localized axisymmetric Ez electric field profile can vary according to the height at which it is calculated relative to the growth surface of the substrate. In accordance with embodiments of the present invention the localized axisymmetric Ez electric field profile is calculated at a height above the growth surface of the substrate of: 4 mm, 6 mm, or 8 mm for a microwave frequency f in a range 400 MHz to 500 MHz; 2 mm, 3 mm, or 4 mm for a microwave frequency f in a range 800 MHz to 1000 MHz; or 0.7 mm, 1.0 mm, or 1.5 mm for a microwave frequency f in a range 2300 MHz to 2600 MHz. It has been found that the electric field profile is significantly perturbed when a substrate is introduced into the resonance cavity as can be shown by modelling or empirical measurement. In this regard, Figures 1(a) to 1(c) illustrate electric field profile plots showing how the electric field varies with differing height of a substrate within a resonance cavity of a plasma reactor. The plots show the magnitude of the electric field Ez on the Y-axis against the lateral position X across the diameter of the resonance cavity above the substrate. Figure 1(a) illustrates the electric field profile when the growth surface of the substrate S is located just above a base B of the resonance cavity C. The electric field profile is dominated by that of the empty chamber which is a Jo Bessel function for a TMoin chamber. There is only a slight contribution to the electric field magnitude from the upper edge of the substrate forming a coaxial mode set up between the substrate and the chamber wall. In this arrangement, the electric field is high above a central region of the substrate and drops off significantly towards the edge of the substrate. As such, this electric field profile results in poor CVD diamond growth in a peripheral region of the substrate growth surface. Figure 1(b) illustrates the electric field profile when the growth surface of the substrate S is located high above the base B of the resonance cavity C. The electric field profile is now dominated by the coaxial mode set up between the substrate and the chamber wall which decays evanescently into a central region of the chamber. In this arrangement, the electric field is high above a peripheral region of the substrate and drops off towards the central region of the substrate. As such, this electric field profile results in poor CVD diamond growth in a central region of the substrate growth surface. Figure 1(c) illustrates the electric field profile when the growth surface of the substrate S is located at the correct height above a surrounding surface within the resonance cavity C. The electric field profile of the empty chamber is balanced with the coaxial mode set up between the substrate and the chamber wall to form a substantially uniform electric field region over the majority of the substrate with a ring of higher electric field localized around the substrate edge. The central region of the electric field is substantially uniform but has a slightly lower electric field region just inside the ring of higher electric field localized around the substrate edge. One would think that this lower electric field region would lead to poor CVD diamond growth at this region of the growth surface. However, in practice it has been found that the higher electric field ring immediately outside the region of lower electric field aids in pulling the plasma outwards, compensating for the slight non-uniformity in the central region and resulting in a large, flat, uniform plasma over the majority of the substrate enabling uniform CVD diamond growth over large areas. It should be noted that while the electric field profile is a property present when the microwave plasma reactor in use, it is also uniquely defined when not in use by modelling the microwave plasma reactor's electric field profile either at its resonant frequencies, or when modelled as being present if driven by a given frequency. Either of these models may be applied to a microwave plasma reactor to determine its electric field profile without undue burden. While the first aspect of the present invention has been described above in relation to the electric field profile, which requires modelling (e.g. modelled at resonance) or empirical measurement to determine, a more simplistic definition may be given in terms of simple dimensional data for the substrate and its location within the resonance cavity of a plasma reactor. In practice, the present inventors have found that a large, flat, uniform plasma over the majority of the substrate enabling uniform CVD diamond growth over large areas can be achieved when a ratio of substrate diameter / height of the growth surface of the substrate is in a range 10 to 14, 11 to 13.5, or 11.0 to 12.5, wherein the height of the growth surface of the substrate is relative to a mean height of a surface surrounding the substrate. Accordingly, this alternative definition of the first aspect of the invention may be utilized without modelling or empirical measurement. However, if an arrangement falls outside these ranges, it is envisaged that utilizing some alternative chamber geometry it may still be possible to form the electric field profile as previously described. In this case, modelling or empirical measurements may be required in order to confirm whether or not the arrangement conforms with the first aspect of this invention. Conversely, if an arrangement falls outside the definition of the first aspect of the invention based on the electric field profile it may still fall within the alternative definition based on the ratio of substrate diameter / height of the growth surface of the substrate. This may be the case if the dimensions and location of the substrate within the plasma chamber are selected to form an electric field profile as described but some further element or elements are provided to alter the electric field profile above the substrate. For example, as described later a metallic ring may be located around the substrate to reduce the magnitude of the high electric field ring located above the edge of the substrate. Alternatively, or additionally, the substrate holder may be profiled to perturb the electric field in order to reduce the magnitude of the high electric field ring located above the edge of the substrate. Other electric field modifying elements are also envisaged. For example, further metallic bodies which perturb the electric field profile may be located on the substrate holder such as metallic inserts located under the substrate. As such, these arrangements may be configured to have the correct substrate diameter/growth surface height ratio while having an electric field profile without a significant high electric field ring located above the edge of the substrate. For an arrangement in which the substrate holder is the same diameter as the substrate, the substrate holder will be located wholly under the substrate and the surface surrounding the substrate may be formed by the base of the plasma chamber. As such, in this case the mean height of the surface surrounding the substrate will equate to the height of the base B of the plasma chamber C and the height of the growth surface of the substrate, Hgs, will be measured from the base of the plasma chamber surrounding the substrate S and substrate holder SH as illustrated in Figure 2(a). Alternatively, for an arrangement in which the substrate holder is much larger than the substrate thus forming a large flat surface which surrounds the substrate, the mean height of the surface surrounding the substrate will equate to a top surface of the substrate holder. As such, in this case the height of the growth surface of the substrate, Hgs, will be measured from the top surface of the substrate holder SH surrounding the substrate S as illustrated in Figure 2(b). For an arrangement in which the substrate holder extends outwards from the substrate with a sloped, curved, or stepped top surface surrounding the substrate then the mean height of the local surrounding surface, Hiss, can be defined by a mean of a height, Hoca ; of a cross section between the edge of the substrate, at Rs, and a distance approximately two times the thickness of the substrate, 2*Ts, away from the substrate edge, taken in a radial direction, X: j Rs+2Ts 2RS ί Such an arrangement is illustrated in Figure 2(c) for a sloped substrate holder. For example, for a substrate holder having a top surface sloping away from the substrate at an angle of 45° to a distance 2>, 70%, 80%>, 90% or 95% of the diameter of the growth surface. 16. A microwave plasma reactor according to any preceding claim, wherein the substrate is disposed over the supporting surface of the substrate holder and spaced apart by spacer elements to form a gas gap having a height h between the supporting surface of the substrate holder and a rear surface of the substrate, and the microwave plasma reactor further comprising a gas supply system for supplying gas to said gas gap, the spacer elements being configured to define a central gas gap cavity under the substrate in which gas from the gas supply system can pool. 17. A microwave plasma reactor according to claim 16, wherein the height h of the gas gap is in the range 25 μπι to 2000 μηι, 50 μπι to 1000 μηι, 100 μιη to 750 μηι, 500 μπι to 750 μηι, 600 μπι to 650 μηι, 100 μπι to 300 μηι, or 150 μπι to 250 μπι. 18. A microwave plasma reactor according to any one of claims 13 to 17, wherein the substrate temperature control system further comprises a temperature modifying ring disposed around the substrate to cool the peripheral region of the growth surface of the substrate. 19. A microwave plasma reactor according to claim 18, wherein the temperature modifying ring is formed by providing a profile in the supporting surface of the substrate holder around the substrate or by providing a separate component disposed over the substrate holder. 20. A microwave plasma reactor according to claim 19, wherein the temperature modifying ring is disposed on spacer elements over the substrate holder. 21. A microwave plasma reactor according to any one of claims 18 to 20, wherein the temperature modifying ring has a melting point greater than 500°C and a thermally conductivity greater than 10 Wm^K"1. 22. A microwave plasma reactor according to any one of claims 18 to 21, wherein the temperature modifying ring is metallic. 23. A microwave plasma reactor according to claim 22, wherein the temperature modifying ring is made of tantalum, molybdenum, tungsten, or an alloy thereof. 24. A microwave plasma reactor according to any one of claims 18 to 23, wherein the temperature modifying ring has a sloped outer surface. 25. A microwave plasma reactor according to claim 16 or 17, wherein the height h of the gas gap varies by no more than 200 μηι, 150 μπι 100 μηι, 80 μηι, 60 μπι, 40 μπι, 20 μπι, 10 μπι, or 5 μπι across at least a central region of the substrate having a centred diameter equal to or greater than 60%, 70%, 80%, 90%, 95%, or 99% of a total diameter of the substrate. 26. A microwave plasma reactor according to claim 16, 17 or 25, wherein the gas gap has a central region with a first gas gap height and a peripheral region with a second gas gap height, the first gas gap height being larger than the second gas gap height. 27. A microwave plasma reactor according to any preceding claim, wherein the substrate holder is configured to be removable from the plasma chamber. 28. A substrate for use in a microwave plasma reactor according to any preceding claim, the substrate comprising: a cylindrical disc of a carbide forming refractory metal having a flat growth surface on which CVD diamond is to be grown and a flat supporting surface opposed to said growth surface, wherein the cylindrical disc has a diameter of 80 mm or more, wherein the growth surface has a flatness variation no more than 100 μηι, and wherein the supporting surface has a flatness variation no more than 100 μπι. 29. A substrate according to claim 28, wherein the flatness variation of the growth surface is no more than 75 μηι, 50 μπι, 40 μπι, 30 μηι, 20 μπι, 10 μπι, 5 μπι, or 1 μπι. 30. A substrate according to claim 28 or 29, wherein the flatness variation of the supporting surface is no more than 75 μηι, 50 μηι, 40 μηι, 30 μηι, 20 μπι, 10 μπι, 5 μπι, or 1 μπι. 31. A substrate according to any one of claims 28 to 30, wherein the carbide forming refractory metal is selected from one of molybdenum, tungsten, niobium, or alloys thereof. 32. A substrate according to any one of claims 28 to 31, wherein the cylindrical disc comprises no more than 0.5%, 0.1%, 0.075%, 0.05%, 0.025%, 0.01%, 0.005%, or 0.001%) by weight of graphite forming impurities at the growth surface. 33. A substrate according to any one of claims 28 to 32, wherein the cylindrical disc is formed of at least 99%, 99.5%, 99.9%, 99.95%, or 99.99% by weight of a carbide forming refractory metal. 34. A substrate according to any one of claims 28 to 33, wherein the growth surface has a surface roughness Ra in the range 1 nm to 1 μπι, 1 nm to 500 nm, 10 nm to 500 nm, 10 nm to 200 nm, 10 nm to 100 nm, lOnm to 50 nm, 20 nm to lOOnm, or 50 nm to 100 nm. 35. A substrate according to any one of claims 28 to 34, wherein an edge of the substrate around the growth surface is one of: sharp; chamfered; or rounded. 36. A substrate according to any one of claims 28 to 35, wherein a circular trench is provided in the growth surface separating an edge of the substrate growth surface from a central region. 37. A method of manufacturing synthetic diamond material using a chemical vapour deposition process, the method comprising: providing a reactor configured for manufacturing synthetic diamond material; locating a substrate over a substrate holder within the reactor, the substrate comprising a growth surface on which synthetic diamond material is to be grown; feeding process gases into the reactor; and growing synthetic diamond material on the growth surface of the substrate, wherein the method further comprises: taking at least two temperature measurements, including one or more measurements in a central region of the growth surface of the substrate and one or more measurements in a peripheral region of the growth surface of the substrate during growth of the synthetic diamond material; and controlling a temperature difference between the central region and the peripheral region of the growth surface of the substrate during growth of the synthetic diamond material based on the at least two temperature measurements, wherein the temperature of the growth surface of the substrate during growth of the synthetic diamond material is controlled to fulfil the condition 5°C < Tc - Te < 120°C, where Tc is a temperature in the central region of the growth surface of the substrate and Te is a temperature in the peripheral region of the growth surface of the substrate. 38. A method according to claim 37, wherein temperature of the growth surface of the substrate is controlled during to fulfil the condition: 10°C < Tc - Te < 100°C; 10°C < Tc - Te < 80°C; 20°C < Tc - Te < 80°C; or 20°C < Tc - Te < 60°C. 39. A method according to claim 37 or 38, wherein the reactor is operated at a power density in the range 0.05 to 10 W/mm2 or 1 to 5 W/mm2 of the growth surface of the substrate. 40. A method according to any one of claims 37 to 39, wherein the reactor is operated at a pressure equal to or greater than: 140, 150, 180, or 200 Torr at a microwave frequency in a range 2300 to 2600 MHz; 80, 100, 120, 140, or 160 Torr at a microwave frequency in a range 800 to 1000 MHz; or 30, 40, 50, 60, or 70 Torr at a microwave frequency in a range 400 to 500 MHz. 41. A method according to any one of claims 37 to 40, wherein the substrate is disposed over the substrate holder and spaced apart by spacer elements to form a gas gap, wherein gas is supplied to the gas gap, and wherein a flow rate of said gas is controlled to be no more than 5%, 4%, 3%, 2%, or 1% of a flow rate of the process gas fed into the reactor. 42. A method according to claim 41, wherein the gas supplied to the gas gap comprises as least two gases having different thermal conductivities and the ratio of the at least two gases is varied to control the temperature difference between the central region and the peripheral region of the growth surface of the substrate during growth of the synthetic diamond material based on the at least two temperature measurements 43. A method according to claim 41 or 42, wherein the gas supplied to the gas gap is composed of gas types which are also fed into the reactor as process gases. 44. A method according to any one of claims 37 to 43, wherein the synthetic diamond material is grown to form a polycrystalline diamond wafer having a diameter in the range: 165 mm to 415 mm, 185 mm to 375 mm, 205 mm to 375 mm, 205 mm to 330 mm, or 240 mm to 330 mm for a microwave frequency f in the range 400 to 500 MHz; 80 mm to 200 mm, 90 mm to 180 mm, 100 mm to 180 mm, 100 mm to 160, or 115 mm to 160 mm for a microwave frequency f in the range 800 to 1000 MHz; or 30 mm to 75 mm, 33 mm to 65 mm, 37 mm to 65 mm, 37 mm to 58 mm, or 42 mm to 58 mm for a microwave frequency f in the range 2300 to 2600 MHz, and wherein the polycrystalline diamond wafer is spontaneously delaminated from the substrate on cooling after the chemical vapour deposition process is completed to yield a free-standing polycrystalline diamond wafer which is substantially crack free over at least a central region thereof, wherein the central region is at least 70%, 80%, 90%, or 95% of a total area of the free-standing polycrystalline diamond wafer, and wherein the central region has no cracks which intersect both external major faces of the free-standing polycrystalline diamond wafer and extend greater than 2 mm in length. 45. A method according to claim 44, wherein the polycrystalline diamond wafer is grown to a thickness of at least 100 μηι, 300 μηι, 500 μηι, 700 μιη, 1.0 mm, 1.2 mm, 1.5 mm, 2.0 mm, or 2.5 mm. 46. A method according to any one of claims 37 to 45, wherein single crystal diamond substrates are mounted over the substrate for growing single synthetic crystal diamond material thereon and the at least two temperature measurements are taken at areas of the substrate between the single crystal diamond substrates. 47. A method according to any one of claims 37 to 46, wherein the substrate is reprocessed between synthetic diamond growth runs to maintain a rear surface profile. 48. A method according to any one of claims 37 to 47, wherein the substrate holder is re-processed between synthetic diamond growth runs to maintain a profile of a supporting surface of the substrate holder. 49. A method according to claim 47 or 48, wherein a height of the substrate within the reactor is adjusted, when necessary, between synthetic diamond growth runs to account for material removed from the substrate and/or substrate holder by reprocessing and maintain a substantially constant height of the growth surface of the substrate within the reactor during subsequent synthetic diamond growth runs utilizing the same substrate, wherein the height of the growth surface is maintained within 2 mm, 1 mm, 0.8 mm, 0.5 mm, 0.3 mm, or 0.2 mm of a target height of the growth surface of the substrate within the reactor. 50. A method according to any one of claims 37 to 49, wherein the reactor is inverted whereby a base of the reactor supporting the substrate forms an upper wall of the reactor relative to earth. 51. A method of manufacturing synthetic diamond material using a chemical vapour deposition process, the method comprising: providing a reactor configured for manufacturing synthetic diamond material; locating a substrate over a substrate holder within the reactor, the substrate comprising a growth surface on which synthetic diamond material is to be grown; feeding process gases into the reactor; and growing synthetic diamond material on the growth surface of the substrate, wherein the synthetic diamond material is grown to form a polycrystalline diamond wafer having a diameter of at least 120 mm, and wherein the polycrystalline diamond wafer is spontaneously delaminated from the substrate on cooling after the chemical vapour deposition process is completed to yield a free-standing polycrystalline diamond wafer which is substantially crack free over at least a central region thereof, wherein the central region is at least 70% of a total area of the free-standing polycrystalline diamond wafer, and wherein the central region has no cracks which intersect both external major faces of the free-standing polycrystalline diamond wafer and extend greater than 2 mm in length. 52. A method according to claim 51, wherein the central region is at least 80%, 80%), 90%), or 95%) of a total area of the free-standing polycrystalline diamond wafer 53. A method according to claim 51 or 52, wherein the polycrystalline diamond wafer has a diameter of at least 140 mm, 160 mm, 200 mm, or 250 mm, 54. A method according to claim 53, wherein the polycrystalline diamond wafer has a diameter no more than 400 mm or 300 mm. 55. A method according to any one of claims 51 to 54, wherein the polycrystalline diamond wafer is grown to a thickness of at least 1.0 mm, 1.2 mm, 1.5 mm, 2.0 mm, or 2.5 mm. 56. A method according to any one of claims 51 to 55, wherein the reactor is a microwave plasma reactor.