The present invention relates to a method of pressure treatment, and particularly but not exclusively a method of pressure treatment to modify an item. Lubricant is applied to engine components to improve resistance to corrosion and reduce wear. In the case of gas turbine engine components, dry film lubricant is commonly applied to the components. During engine repair it is required to thoroughly remove the dry film lubricant, including from any cracks, in order to inspect the component and in particular, to carry out crack inspection. Complete removal of the dry film lubricant is extremely difficult especially from within cracks and is therefore time consuming. Conventionally, following mechanical removal of the bulk of the lubricant, chemical techniques such as dipping in an acid or alkali bath are used to remove the dry film lubricant. For example, the process for a titanium component includes dipping the component in an acid bath followed by a wash, degreasing with a solvent and drying by air blast to prepare the component for inspection. Such techniques, however, also remove some of the surface of the component, reducing the life of the component due to hydrogenation, and take a significant time to carry out. The use of chemicals such as acids or alkalis leads to the problems of safe environmental disposal of the used chemicals . According to the present invention, there is provided a method of pressure treatment of an item, the method comprising the steps of subjecting at least part of the item to a gas at a first pressure, and thereafter reducing the pressure of said gas to a second pressure at a sufficient rate to modify the item. Preferably, the said part of the item is positioned within a container. Preferably the first pressure is provided by a pressure creating means. Preferably the reduction in pressure from the first pressure to the second pressure is provided by a pressure reducing means. According to another aspect of the present invention, there is provided apparatus for the pressure treatment of an item, the apparatus comprising a container, at least part of the item being positionable within the container, a pressure creating means to provide a gas at a first pressure within the container, and a pressure reducing means operable to reduce the pressure experienced by the said part of the item from the first pressure to a second pressure at a sufficient rate so that the item is modified. Preferably, the item is modified by the widening of any crack defects in the item. Preferably, the widening of the crack defects results in the destruction of the item. Preferably, the gas is maintained at the first pressure for a sufficient time to allow the gas to diffuse into the crack defects. Preferably the rate of pressure reduction allows the gas within the crack defects to expand to widen the crack defects . Alternatively or additionally, the item may be modified by the removal of at least part of a surface coating from the said part of the item. Preferably, the gas is maintained at the first pressure for a sufficient time to allow some of said gas to diffuse between at least some of the coating of the item. The rate of reduction of the pressure of the gas may be sufficient to allow gas between the coating and the item to expand and remove the coating from the item. Preferably, the difference between the first pressure and the second pressure is not less than 2 bar. Preferably the reduction in pressure from the first pressure to the second pressure takes place in a time period of 0.25 seconds or less. The gas may be air. The second pressure may be below atmospheric pressure . The container may be a pressure vessel, and the pressure creating means may comprise a compressor or pump. In one embodiment, the pressure reducing means may comprise an aperture defined in a wall of the container, and the reduction in pressure from the first pressure to the second pressure may be provided by permitting the gas to escape through the aperture. The pressure reducing means may comprise a valve in communication with the aperture, and may further comprise a vacuum or suction pump. In a second embodiment, the pressure reducing means may comprise an aperture defined in a wall of the container, and the reduction in pressure from the first pressure to the second pressure may be provided by moving the said part of the item from within the container to outside the container through the aperture. Preferably the shape of the aperture corresponds to the cross-sectional shape of the said part of the item moving through the aperture. A second similarly shaped aperture may be provided to allow the said part of the item to be initially moved from outside the container to within the container. Sealing means may be provided to seal around the said part of the item as the said part moves through the or each aperture. A closure means may be provided to close the or each aperture. In a third embodiment, the container may comprise a plurality of compartments. The plurality of compartments may include a pressure raising compartment, a pressure holding compartment and a pressure reducing compartment, and in use the item may move sequentially from the pressure raising compartment to the pressure holding compartment and then to the pressure reducing compartment . Preferably, the apparatus includes pressure conservation means. Preferably the pressure conservation means is operable in use to contain the escaping gas when the pressure is being reduced, and may be further operable to recycle the escaped gas to the container to increase the pressure in the container when the pressure is being raised. Preferably a means of separating particles of surface coating from the gas is provided. The separating means may comprise any of a filter, a scrubber, a cyclone or an electro-static precipitator. Preferably the surface coating includes a degreasing material, and an applicator may be provided to apply the degreasing material to the said part of the item prior to the reduction in pressure from the first pressure to the second pressure. The applicator may apply the degreasing material by spraying or dipping. The surface coating may include an anti-corrosion coating. The surface coating may include a lubricant coating, which may be a dry film lubricant. The item may be a manufactured component, which component may be an aerospace component . Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:- Fig. 1 is a schematic representation of a process for the pressure treatment of an item; Figs. 2A and 2B are expanded schematic representation of the process of Fig. 1; Fig. 3 is a schematic representation of another process for the pressure treatment of an item; Figs. 4A and 4B are schematic representations of part of another process for the pressure treatment of an item; and Fig. 5 is a schematic representation of another process for the pressure treatment of an item. Fig. 1 shows apparatus 10 for the pressure treatment of an item 16, the apparatus comprising a container 12, the item 16 being positioned within the container 12. A pressure creating means 18 provides a gas at a first pressure within the container 12 and a pressure reducing means 20 is operable to reduce the pressure from the first pressure to a second pressure at a sufficient rate so that the item 16 is modified. The pressure creating means 18 is in communication with a gas supply, the gas intake to the pressure creating means being indicated by arrow A. The gas is exhausted from the pressure reducing means as indicated at arrow B. In one example according to the invention, the item 16 comprises a surface coating, and when the pressure is reduced, the surface coating is at least partially removed from the item 16. In another example according to the invention, the item 16 comprises a crack defect, and as the pressure is reduced the crack defect is widened, causing degradation or destruction of the item 16. The gas is maintained at the first pressure for a sufficient time for the gas to diffuse into the crack defect, and when the pressure is reduced, the gas in the crack expands rapidly, widening the defect. Fig. 2A shows an expanded representation of the process of Fig. 1 for the removal of a surface coating from an item. The apparatus 110 comprises a first container in the form of a pressure vessel 112, in which an item in the form of an aero engine component 116 is positionable. The aero engine component 116 is coated with a dry film lubricant. A pressure creating means in the form of a compressor 118 takes in air as indicated by arrow A to provide air at a first pressure within the pressure vessel 112. An inlet valve 117 is provided to isolate the pressure vessel 112 from the compressor 118. A pressure reducing means comprises an aperture 119 defined in a wall of the pressure vessel 112 and an outlet valve 120 in communication with the aperture 119 and operable to reduce the pressure from the first pressure to a second pressure so that the surface coating is at least partially removed from the item 116. The apparatus 110 further comprises a second container in the form of a degreaser applicator 126. The degreaser applicator 126 applies a degreasing material 128 to the component 116 by spraying, prior to the component 116 being positioned in the pressure vessel 112. Air exhausted from the outlet valve 120 enters a separator 122 to separate particles of surface coating released from the component 116 from the air stream. The separator 122 may comprise any suitable known separation means, such as an electrostatic precipitator, a filter, a scrubber or a cyclone, or a combination of these. From the separator 122, the air stream enters a suction or vacuum pump 124. The suction or vacuum pump 124lowers the second pressure below atmospheric pressure. This allows either a larger pressure differential to be achieved, or allows the same pressure differential to be achieved with a lower first pressure, which thus allows a lower specification pressure vessel and compressor to be used with a consequent reduction in cost and a reduction in cycle time for the process. Air is exhausted from the process as indicated at arrow B. In use, the component 116 is placed in the degreaser applicator 126 and degreasing material 128 is applied to the surface of the component 116. The component 116 is then transferred to the pressure vessel 112. The inlet valve 117 is opened and the outlet valve 120 is closed. The compressor 118 provides compressed air to raise the pressure of air in the pressure vessel 112, until the first pressure is reached. The first pressure may be maintained for a period. The inlet valve 117 is then closed and the compressor ceases to operate. The outlet valve 120 is then opened, permitting air to escape through the aperture 119, causing a rapid reduction in pressure in the pressure vessel 112 which in turn causes the dry film lubricant surface coating to be removed, the removed surface coating also including the degreaser. The air exhausted from the pressure vessel 112 passes through the outlet valve 120 and into the separator 122, where it is substantially cleaned of the particles of removed surface coating. The clean air is then drawn by the suction/vacuum pump 124 and exhausted from the process. The suction/vacuum pump 124 only operates when the outlet valve 120 is open. The values for the first and second pressures and the rate of reduction of the pressure are dependent on the material of the component and the characteristics of the surface coating of the component. In the case of a light, dry powder coating, the coating can be removed by a first pressure of between 3 and 5 bar and a second pressure of 1 bar, the reduction in pressure being over a period of substantially 0.25 seconds. Further variables affecting the process are the rate of pressure increase up to the first pressure, and the time period for which the pressure is held at the first pressure. In the case of the light powder coating, the air pressure was raised from one bar to between 3 and 5 bar over a period of between 2 - 5 minutes, and the pressure was reduced as soon as the first pressure was achieved. Surface coating removal of oil based coatings is improved by utilising a higher first pressure, a longer time period at the first pressure, and a larger pressure reduction. As an example, a pressure reduction of 10 - 15 bar over 0.25 seconds can be used to remove an oil based surface coating. The removal of light solvent surface coatings is similarly improved at higher first pressures and larger pressure reductions. For example, a light solvent coating can be removed by a pressure reduction of 40 - 50 bar over a period of substantially 0.25 seconds. The efficiency of removal is also improved as the time period of the pressure reduction is reduced. A side effect of the rapid reduction in pressure is the widening of any crack defects in the component 116. This can lead to the breakage or destruction of the component. Since, for engine components, and in particular gas turbine components, the aim of the inspection process is to identify any crack defects in a component, this side effect is advantageous. Fig 2B shows the process of Fig 2A as described above, modified to include pressure conservation means in the form of a pressure conservation vessel 170, located between the outlet valve 120 and the separator 122. Pressure conservation valve 174 and separator valve 176 allow the air exhausted from the pressure vessel 112 to be selectively directed to either the pressure conservation vessel 170 or the separator 122. In use, when outlet valve 120 is opened, the air is initially directed to the pressure conservation vessel 170, which contains the escaping gas until the pressures in the pressure vessel 112 and the pressure conservation vessel 170 are equalised. The pressure conservation valve 174 is then closed and the separator valve 176 opened, exhausting the air in the pressure vessel 112. The item 116 in the pressure vessel 112 is replaced by a fresh item 116 and the inlet valves 117 and separator valve 176 closed. The outlet valve 120 and the pressure conservation valve 174 are opened, and air flows from the higher pressure in the pressure conservation vessel 170 to the lower pressure in the pressure vessel 112 until the pressures are equalised. The outlet valve 120 and the pressure conservation valve are then shut, the inlet valve 117 opened, and the compressor 118 provides compressed air to raise the pressure of air in the pressure vessel 112. Thus the pressure conservation vessel 170 permits some of the pressure energy in the system to be conserved. Various modifications may be made without departing from the scope of the invention. Although Figs. 1 and 2 show the parts of the process as separate, it will be apparent to the skilled man that these parts could be combined or located relative to each other in a number of ways. For instance, the degreaser spray could be located within the pressure vessel 112. The separator could abut the wall of the pressure vessel, or the separator may form an enclosure around the pressure vessel to act as a containment means. Any suitable container may be used as a pressure vessel. Any suitable pressure creating and pressure reducing means may be employed. Although Fig. 2 shows the degreaser applicator applying the degreaser in the form of a spray, the component 116 may for instance be dipped in a bath of degreasing material. The process could be carried out continuously as described below. Figure 3 shows apparatus 210 for the removal of the surface coating from an item in the form of an elongate component 216. The apparatus 210 comprises a container in the form of a pressure vessel 212 having inlet and outlet apertures 42, 44 defined in walls of the container. The apertures 42, 44 are shaped to correspond with the cross- sectional shape of the component 216 moving through each aperture and seals 40, 46 are provided to seal around the component 216 as it moves through the apertures 42, 44. Closure means in the form of sliding shutters 48, 50 are provided to close the apertures 42, 44 when the component 216 is not present in the apertures. A pressure creating means in the form of a compressor or pump 218 takes in air as indicated by arrow A to provide air at a first pressure within the pressure vessel 212. In use, in an initial condition the shutters 48, 50 close the apertures 42, 44 and the compressor 218 provides air at a first pressure within the pressure vessel 212. The inlet shutter 50 is opened and the component 216 introduced through the aperture 42 into the container 212. The shaped aperture 42 and the seal 40 around the component 216 substantially prevent the loss of pressure from the pressure vessel 212. The component moves through the pressure vessel 212 in the direction shown by arrow C. The part of the component within the pressure vessel 212 is subjected to the gas at the first pressure. After a suitable time period, the component 216 approaches the outlet aperture 44 and the outlet shutter 48 opens, allowing the component 216 through the aperture 44. Again, the shape of the aperture 44 and the seal 46 around the component 216 substantially prevent a loss of pressure within the container 212. As part of the component 216 moves through the outlet aperture 44, the pressure on that part of the component 216 reduces rapidly, causing the surface coating on that part of the component 216 to be removed. Although Figure 3 shows the component 216 as a continuous length between the inlet and outlet apertures 42, 44, the apparatus can be operated with shorter lengths of component 216, with the inlet and outlet shutters 50, 48 arranged to open as the component 216 passes through the respective aperture 42, 44 and to close as soon as the component 216 has passed through the respective aperture 42, 44. As described previously for the process of Fig. 2, the process of Fig. 3 may include degreaser application before entry into the pressure vessel, containment of the released surface coating on exit from the pressure level, a separator, and a vacuum or suction pump to provide a second pressure below atmospheric pressure. Figs. 4A and 4B show schematically in part another example of a continuous process in which a container in the form of a pressure vessel 312 includes a pressure raising compartment 360, a pressure holding compartment 362 and a pressure reducing compartment 364. In use, a component 316 moves through the three compartments in sequence. In the pressure raising compartment 316 the air pressure is raised to a first pressure; the air pressure is maintained at the first pressure in the pressure holding compartment 362, and is reduced to a second pressure in the pressure reducing compartment 364. The pressure raising compartment 360 and the pressure reducing compartment 364 operate as pressure locks so that the pressure in the pressure holding compartment 362 is substantially maintained at the first pressure. The apparatus could be arranged so that the second pressure is below atmospheric pressure as previously described for the processes of Figs. 2 and 3. Fig 4B shows the apparatus of Fig 4A, modified to include pressure conservation means in the form of a communicating passage 372 between the pressure reducing compartment 364 and the pressure raising compartment 360. The communicating passage 372 allows the air escaping from the pressure raising compartment 364 to be utilised in raising the pressure of the air in the pressure raising compartment 360, thus conserving pressure energy. The pressure reducing compartment 314 can be isolated from the pressure raising compartment 360 by valve 374. Figure 5 shows schematically in part another example of a process according to the invention, in which a container in the form of a pressure vessel 412 includes an inner chamber 482 and pressure conservation means in the form of an outer chamber 480, the inner chamber being located within the outer chamber 480. In this example, the pressure of the inner chamber 482 cycles between the first pressure and second pressure as previously described. However, the second pressure is not atmospheric pressure but is a pressure above atmospheric pressure. The pressure in the outer chamber 480 cycles between the second pressure and atmospheric pressure. In use, the pressure in the inner chamber 482 is raised to the first pressure while the outer chamber 480 is at atmospheric pressure, during which time modified items 416 are removed and fresh items 416 are loaded. The inner chamber 482 is then vented so that air escapes from the inner chamber 482 into the outer chamber 480 where it is contained, the pressures of the inner and outer chambers 482, 480 equalising to the second pressure. At this pressure, items 416 can be loaded and unloaded between the inner and outer chambers 482, 480. The inner chamber 482 is then raised to the first pressure and the outer chamber 480 vented to reduce the pressure to atmosphere. This arrangement has a number of advantages. It can be operated batch wise or continuously. The initial pressure differential on venting is maximised since the pressure differential is between the first pressure in the inner chamber 482 and atmospheric pressure in the outer chamber 480, but the pressure energy input required in each cycle is reduced, since energy is only required to raise the pressure of the inner chamber from the second pressure to the first pressure, the second pressure being higher than atmospheric pressure. Thus pressure energy is conserved, while maximising the initial pressure differential at venting. The processes of Figs 3, 4 and 5 could equally be adapted for use to destructively test an item by widening crack defects as described previously. The invention thus provides a method and apparatus by which a surface coating may be removed from an item without requiring either the use of chemicals or by a mechanical means such as brushing or scraping. The process is relatively quick in comparison with present chemical methods and allows the cleaning of complex surfaces which have proved difficult conventionally. The process also allows the easier location of crack defects.