The present invention relates to the field of controlling liquid flow. The control of the flow of water between different water levels is a well known problem. In many parts of the world, including the UK5 sluice gates are used to manage water levels. For example, in the fens sluice gates are used to control water differences of between 1 and 2 metres. A drawback of sluice gates is that they generally use heavy weight mechanical moving parts, which require controlling and servicing and are thus, generally labour intensive to run and maintain. This can be a particularly acute problem in remote areas, where access to the sluice gates is difficult. A further example of sluice gates being used to control water levels is in tidal docks. In tidal docks, the object is to have the gates open for as long as possible to allow shipping access, while trying to avoid the loss of much water from the dock. This is generally done by closing the gates at high tide when the dock is at its fullest. Then, before the approach of a subsequent high tide, sluice gates are opened to drop the water level in the dock by up to a metre. This is to ensure that the levels inside and outside the dock can be equalised near the next high tide level, on the rising tide. Unless these levels are equal, the dock gates cannot be opened against the pressure of the water. This allows deep draught ships to enter and leave the dock from that moment until high tide, when the gates are again closed. This requires careful management of the sluice gates by operators,' who have to predict the height of the next high tide and calculate the quantity of water that has to be released, which partly depends on the number and draught of the ships due to enter and leave the dock. It also requires heavy mechanical moving sluice gates. A first aspect of the present invention provides an apparatus for controlling a flow of liquid between a volume of liquid having an upper surface at a higher level and a volume of liquid having an upper surface at a lower level comprising: a siphon operable to provide fluid communication between said higher level liquid volume and said lower level liquid volume; a gas inlet operable to provide fluid communication between a gas and a fluid within said siphon, said gas inlet being arranged such that fluid flow through said siphon is operable to cause gas at said inlet to be entrained into said fluid flow; and a control system comprising at least one sensor operable to detect a characteristic of said fluid and a valve operable to control an amount of gas flowing through said gas inlet and becoming entrained in said fluid flow, said control system being operable to control a degree of opening of said valve in response to signals received from said at least one sensor, in order to control said flow of liquid. The provision of a siphon allowing liquid to flow from a higher level to a lower level in conjunction with a valve to allow gas to enter the fluid flow, allows for a controllable flow between the higher and lower level. The valve allowing ingress of gas can be used to control the fraction of gas being entrained and carried by the fluid flow. The more gas being entrained in the fluid flow, the greater the work the fluid must do to carry the gas, as it compresses the gas as it carries it down. This takes up some of the head driving the fluid and thus, the fluid travels more slowly. It can also be used to allow a lot of gas to enter and thereby break the siphon suction and stop the fluid flow completely within the siphon. A sensor is used to detect a characteristic of the fluid, and thus the fluid flow can be monitored and the valve controlled to achieve and maintain a desired flow. There are several advantages to controlling the fluid flow in this way. By using a siphon with a valve to control air ingress into the fluid and thereby the amount of liquid flowing between the two levels, heavy mechanical parts requiring manual operation and servicing are reduced and in some cases avoided. Valves are simple robust devices that lend themselves particularly well to remote operation. Furthermore, the entrainment of a gas (often air) into the liquid flow may improve the quality of the liquid (often water). A further point is that the flow of gas created in this manner may itself be used for other purposes. Preferably, said apparatus further comprises a gas driveable engine in fluid communication with said gas inlet, said gas driveable engine being operable to be driven in response to a flow of gas caused by entrainment of gas into said fluid flow. Providing a gas driveable fluid which can be driven by a flow of gas produced by gas becoming entrained in the fluid flow, is an advantageous way of using the siphon not only to control fluid flow between the different levels and thus the height of the higher level, but also to generate useful work. Furthermore, the system is such that the ability to control the valve in response to signals received from the sensor may enable the amount of air being entrained to be varied in response to changes in the system. This can be used to help maintain the flow of gas at a particularly efficient level for driving an engine. A further point is that the suction effect of the air being entrained in the fluid flow means that the output of the gas driveable engine is at a negative pressure. Many gas turbines operate particularly efficiently with a low exhaust pressure. Advantageously, said fluid outlet of said siphon is located at a depth beneath a surface of said lower level liquid volume, said apparatus further comprising a vessel operable to collect gas entrained within said fluid flow substantially at said depth within said lower level liquid, such that gas is at a pressure higher than atmospheric pressure within said vessel, said gas driveable engine being operable to be driven by gas flowing from said vessel to said gas inlet. In some embodiments, rather than driving the engine by a flow of gas directly entrained in the fluid flow, the gas entrained is caught in a vessel at substantially the depth of the outlet (or slightly higher than it), and then the gas from the vessel flows from this point of high pressure to be entrained in the siphon at a lower pressure. The flow of gas can be used to drive the engine. This has the advantage that the pressure difference for driving the gas driveable engine is increased. Most gas driveable engines such as turbines are more efficient if a high pressure difference is used to drive them. The placing of the siphon outlet at a certain depth within the lower level liquid is a simple way of compressing air which can then be used to drive the engine. In some situations the water at the outlet may be shallow and thus, this may not be possible without digging a hole, in such situations this arrangement may not be advantageous. Preferably, said apparatus comprises an engine inlet valve operable to control an amount of air entering said gas driveable engine. The amount of air driving the engine and thus, the amount of power generated and the efficiency of the engine can be controlled by a valve controlling the air input to the engine. This clearly also has an effect on the amount of air entrained in the siphon and thus, on the fluid flowing through the siphon. In some embodiments the control system controls this engine inlet valve to control the amount of gas being entrained and the gas inlet valve can be on or off determining whether air is input to the siphon at all or not. Advantageously, said apparatus comprises a fluid sensor operable to sense said fluid flow at, or downstream from, a point of lowest pressure within said siphon, said fluid sensor being operable to sense at least one of an amount of gas and an amount of liquid present in said fluid flow within said siphon, said controller being operable to control said at least one valve in response to said fluid sensor. The amount of air that can be entrained while not breaking the suction effect of the siphon needs to be carefully controlled as changes in pressure differences and flow rates can affect the amount of air that can be entrained. Thus, an additional sensor to detect the amount of gas present in the flow can significantly increase the efficiency of the system and help prevent undesired interruptions in the fluid flow due to the entrainment of too much air. Furthermore, if a fluid driveable engine is being driven by the gas flow the efficiency of the system can be greatly increased by careful control of the amount of gas being entrained. Furthermore, if the siphon effect is broken and no fluid flow is present such a sensor can be extremely beneficial in any repriming of the siphon. In some embodiments, the apparatus comprises a further gas inlet valve, said further gas inlet valve being operable to admit gas to a high point of said siphon, said control system being operable to control said further gas inlet valve. An additional gas inlet valve can be used to admit a gas to the upper portion of the siphon. This can be used to admit a lot of gas and break the suction effect and thus stop the fluid flow or it can be used to control the amount of gas present in the upper portion of the siphon and thus, affect the amount of fluid flow in this way. This is because the flow rate depends on the relative fractions of the cross section of the top of the siphon which are occupied by air and water. Thus the valve can be used to admit air to the top of the siphon and thus, restrict the flow in this way. This can be particularly important where low fluid flows are required. In such cases controlling the fluid flow by amount of air entrained may not be practical and doing it by admitting a gas pocket to the top of the siphon may be more effective. Advantageously, the apparatus comprises a pump in fluid communication with a portion of said siphon at or downstream from a point of lowest pressure within said siphon and operable to lower the pressure in the siphon so as to cause fluid to flow from a siphon inlet at a higher liquid level volume to a siphon outlet at a lower liquid level volume. A pump is provided in order to initiate fluid flow in the siphon at the start of operations or following an interruption in the fluid flow. This means the fluid flow can be restarted at will. Preferably, said apparatus comprises a further conduit, said further conduit providing fluid communication between said liquid in said higher level liquid volume and said liquid in said lower level liquid volume, said further conduit comprising a valve, said pump being operable to be driven in response to a fluid flow in said further conduit. The provision of a fluid flow driven pump and an additional conduit such that flow of liquid from the higher level to lower level volume can be used to drive the pump, obviates the need of an additional external power source for the pump. In preferred embodiments, said apparatus further comprises at least one venturi, said gas inlet arranged such that said gas within said inlet is operable to be entrained into said fluid flowing through said venturi. Preferably, a venturi is provided within the siphon to cause acceleration of the fluid flow. This can help to increase the suction exerted on the gas being entrained within the fluid flow. It can also help to provide a stable system. Air admission to a siphon by itself tends to be hard to control, as the siphon has a negative output impedance characteristic due to the compression of the water by the incoming air. This means that the water has to accelerate, creating a venturi effect which increases the pressure drop. Venturis, on the other hand, have a strong positive output impedance characteristic, such that when air is admitted the suction pressure falls. Therefore a combination of the two techniques allows for the construction of a well- behaved system. A further example where it may be particularly advantageous is if the apparatus were used to control a flow of liquid between two sides of a barrier in a tidal flow system, where the rate of flow is lowest when the difference in heights is lowest. The suction effect from the siphon is highest when the upper water level is lowest, however, the suction effect due to venturi is lowest when the flow rate is lowest. Thus, the suction loss due to the decreasing flow in the venturi can be at least partially compensated for by an increase in suction effect due to the siphon. A similar compensation of effects can occur in regions where waves may be present. In a siphon according to an embodiment of the invention, the contribution to the total (siphon + venturi) suction from the siphon is determined by the difference between the height of the top of the siphon and the upper water level. This is because the effective density of the water/air mix in the downward leg is less than in the (water only) upward leg, so the negative hydrostatic pressure at the top of the siphon is about the same for each leg. Hence, for example, a wave that increases the water level on the high side, but not on the low side, increases the level difference, increases the flow rate, increases the suction from the venturi (which depends on the square of the flow rate) but decreases the suction contributed by the siphon. Thus there is again at least some compensation between the different effects and this helps to stabilise the system. Preferably, said apparatus comprises a plurality of gas inlets, said plurality of gas inlets comprising a plurality of slots arranged within a wall at or downstream of an area of minimum cross section of said at least one venturi. The use of slots downstream of the throat of the venturi has the result that when air is admitted, it will first start being entrained just behind the leading edges of the slots, because that is where the suction is greatest. The admission of this air will, however, deprive the water of some of its flow cross section thereby increasing its speed, this, combined with the suction suppressing effect of admitting the air increases the suction further along the slot, encouraging more air to be admitted there. Advantageously, said slots are staggered such that slots located at a higher region of said siphon are located further downstream of said area of minimum cross section of said venturi, than slots at a lower region of said venturi are. The suction developed by the venturi is maximum where the venturi throat cross section is least. Hence, the location of the starting point of a slot relative to the throat determines the maximum suction that can be developed by that particular slot. In wider siphons there will therefore be a hydrostatic pressure difference between the top and the bottom of the pipe. By staggering the leading edges of the slots according to vertical location, the venturi system can be designed to develop a bigger suction at the bottom of the pipe, where the hydrostatic pressure is highest and air entrainment is hardest, than at the top, where the hydrostatic pressure is least, and additional venturi suction is least needed. In this way, compensation for the hydrostatic pressure difference in the pipe can be obtained. In some embodiments, said apparatus further comprises a plurality of siphons arranged in parallel and a duct, each siphon having a gas inlet providing fluid communication between said duct and said plurality of siphons, said gas inlets being in fluid communication with one another via said duct. The system lends itself well to parallel operation. This means that a large amount of liquid such as water may be conducted through a number of pipes or the siphons arranged in parallel. Thus, a size of pipe suitable for a particular application can be selected and the number used varied depending on the desired amount of flow. Furthermore, a parallel system allows for an additional level of fluid flow control as the number of siphons actually operational and conducting a flow of liquid can be changed to suit circumstances and the desired flow. Thus, if certain siphons operate more efficiently at a particular flow rate this rate can be maintained and the overall flow varied by turning on and off a certain number of siphons. Preferably, said plurality of siphons each comprise a venturi, each of said plurality of outlets being located within said at least one venturi such that said gas within said conduit is operable to be entrained into said fluid flowing through said Venturis. Venturis are also highly advantageous in the parallel siphon system. Furthermore, in a water driven venturi, the pressure drops in proportion to the square of the water speed through the venturi. When air is admitted to this device, the suction drawing in the air falls, more or less in proportion to the amount of air admitted. There are at least two reasons for this. In the first place, admitting the air increases the resistance to the flow. Therefore, if the driving head is fixed, the flow rate goes down, the speed goes down, and the suction gets less. Secondly, as more air is admitted, the air injection system (slots in this case) can see less and less of the fast flowing water, so can experience less of the available suction. Hence, Venturis in parallel represent a very stable suction system. As soon of one of them gets too much air, its pulling power falls, so it gets less air. The system is self-stabilising. • Preferably, the pump comprises one of said siphons, fluid flow in said one of said siphons being operable to entrain air from said conduit thereby reducing pressure within said duct and via said gas inlets in said plurality of siphons. Although a pump can be used to reduce air pressure within the duct and initiate flow in some of the siphons, preferably one of the siphons is used to do this. This avoids the need for a large separately powered pump and is a further advantage of having parallel siphons. A small pump is generally kept available in order to evacuate the priming siphon in case it has a small air leak or needs to be shut down for servicing. Furthermore, having a siphon continuously flowing and entraining gas may in some embodiments increase aeration of the liquid. In some embodiments, said at least one sensor comprises a fluid flow sensor operable to monitor a speed of said fluid flow through said siphon. The speed of fluid flow through the siphon is important when controlling the liquid levels and knowledge of this can be used to control the valves and thus the liquid flow. In other embodiments said least one sensor comprises a fluid inlet sensor operable to monitor fluid pressure adjacent said fluid inlet of said siphon. The fluid pressure difference between the inlet and outlet of the siphon is also an important factor in determining fluid flow within the siphon and monitoring it can be used to control the valves and achieve a desired liquid flow. Preferably, said controller is operable to control said valve in response to signals from each of said sensors. In some systems more than one sensor is used and the system is controlled in response to several signals. In some embodiments a fluid outlet sensor is helpful to measure the pressure difference between the inlet and outlet of the siphon, and also possibly to provide an indication of the pressure of gas in a vessel at this level, while in other embodiments the outlet is directly to atmosphere and such a sensor is not necessary. Preferably, said liquid is water and said gas is air. In most embodiments the flow that is to be controlled is a flow of water between different levels. It makes sense in such circumstances to entrain air within the water. Air is readily available and furthermore, its entrainment in water may increase the quality of the water thereby providing an additional advantage of the system. Advantageously, said apparatus comprises a pipe having a loop in fluid communication with an inlet side of said gas driveable engine, said gas entering said gas driveable engine being caused to flow around said loop prior to entry into said gas driveable engine such that any non gas particles within said gas are thrown to an outside of said loop, said loop comprising a drain such that at least some of said non gas particles exit said loop via said drain. Exerting a centrifugal force on the gas before it enters the engine can cause non-gas particles to be thrown to the outside of the loop where they may be drained. This can be particularly helpful where the liquid is sea water and the gas is air containing sea spray. Sea water is very corrosive and it is highly beneficial to be able to clean it from the air prior to the air entering the engine. Preferably, said apparatus comprises a battery; said gas driveable engine being operable to generate electricity, at least a portion of said generated electricity being operable to recharge said battery, said control system being operable to be powered by said battery. The use of a local rechargeable battery enables the valves and sensors to be operated in remote locations without the need for an external power supply or the continual changing of batteries. Preferably, said apparatus further comprises a microprocessor connected to a mobile phone, said microprocessor being operable to monitor and report the status of said at least one valve and said at least one sensor. Such an arrangement allows for remote operation. Preferably, said battery is operable to power said microprocessor and mobile phone. Owing to the remote location of many of these systems, the ability to monitor and control them remotely is extremely advantageous. A further aspect of the present invention provides a plurality of apparatus according to a first aspect of the present invention, and a microprocessor connected to a mobile phone, said microprocessor being operable to monitor and report the status of said valves and sensors and comprising a central control computer for monitoring and controlling said plurality of apparatus in response to signals received from said plurality of mobile phones In some situations such as in the fens, a plurality of such apparatus could be constructed across a plurality of sluices. Central control would allow not only the power generation to be monitored and controlled but also water levels within the various basins to be monitored and centrally controlled. A still further aspect of the present invention provides a method of controlling a flow of liquid between a volume of liquid having a surface at a higher level and a volume of liquid having a surface at a lower level, comprising: arranging a siphon between said higher level liquid and said lower level liquid, said siphon having a gas inlet; causing said liquid at said higher level to flow through said siphon to said liquid at said lower level; sensing at least one characteristic of a fluid flowing through said siphon; controlling a valve to vary a quantity of gas admitted into the siphon through a gas inlet, said gas being entrained in said fluid flow, in response to signals received from said at least one sensor, in order to control said flow of liquid. Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 schematically shows an apparatus for controlling a flow of liquid between a liquid at a higher level and a liquid at a lower level; Figure 2 schematically shows a unidirectional flow apparatus like that of Figure 1; Figure 3 schematically shows an apparatus for controlling a flow of liquid between a liquid at a higher level and a liquid at a lower level having slanted siphon legs; Figure 4 schematically shows a section of an apparatus for controlling a flow of liquid; Figure 5 schematically shows an apparatus for controlling a flow of liquid having air collection tanks; Figure 6 shows a plurality of siphons for controlling liquid flow according to an embodiment of the invention; Figure 7 schematically shows a gas inlet to a siphon of figure 6; Figures 8a and 8b show a centrifugal air inlet to a turbine for use with an embodiment of the present invention; Figure 9 shows a control system for controlling valves in an embodiment of the present invention; and Figure 10 shows a siphon according to an embodiment of the present invention. Figure 1 schematically shows an apparatus for controlling a flow of liquid between a liquid at a higher level 10 and a liquid at a lower level 20. The apparatus is shown very schematically and in reality the siphon would have rounded bends. The apparatus comprises a siphon 30 with a gas inlet 40 operable to admit air into the flow of water. The siphon has at least one venturi 50 at the gas inlet 40 operable to accelerate the fluid flow as it passes the inlet. Flow can be drawn into the siphon 30 by extracting air through the gas inlet 40 which sucks water up from the siphon inlet 32. When the water reaches the top of the siphon it will then fall under gravity towards a siphon outlet 34. Thus, a flow of water will be initiated and this will continue to flow driven by the water level difference between levels 10 and 20. The flow of water within this siphon will entrain air at the gas inlet 40 into the flow and thus a water and air mixture will flow in the latter half of the siphon. The gas inlet 40 has a valve 60 to control the air entering the siphon. This may be a simple on/off valve either to admit air or not, or it may be a more controllable valve which can control the amount of air admitted. The amount of air admitted affects the flow rate of the water and can be used to control the amount of water that flows from liquid level 10 to liquid level 20. Thus, the height of liquid level 10 can be controlled by the flow rate in the siphon. Figure 1 is an apparatus that is suitable to control liquid flow in either direction. Thus, the flow may be due to tidal changes with level 10 or 20 being the higher level depending on the time of the tide. Figure 2 shows a unidirectional apparatus similar to that of Figure 1, but suitable for use where the flow is always in one direction, for example in a river. In this embodiment, the gas inlet 40 is on the lower half of the siphon and the Venturis are there as well. It has been found that this is more efficient place to put the gas inlet and Venturis and therefore in a unidirectional flow situation this is how the apparatus is arranged. Figure 3 shows an embodiment where the legs of the siphon are not straight. It is possible for the siphon 30 to have curved corners and in fact this is generally the case and for the legs of the siphon to be at an angle. Furthermore the inlet 32 and outlet 34 do not have to be at the same height. In preferred embodiments the siphons have fluid directing formations (not shown) within the downleg of the siphon operable to cause the water flowing in the downleg of the siphon to spin 31. This is advantageous in a vertical downleg (such as that shown in Figure 1) as it centres the air keeping it mid stream and away from the slower moving water at the walls. In a sloping downleg as shown in Figure 3, it continually moves any air that has accumulated inside the upper wall of the pipe down to the bottom again, i.e. it maintains the mixing of the air with the water, thereby ensuring proper transmission of the air to the output. Figure 4 shows a portion of an alternative embodiment, where the air entrained in the fluid flow is collected in a tank located close to outlet 34. This tank is at a similar depth underwater to outlet 34 and thus the air is under pressure P at this point. This is a convenient way of collecting compressed air, the flow of the fluid entrains the air as it passes the gas inlet and acts to compress the air as it draws it down the siphon leg towards siphon outlet 34. Clearly compressed air contains energy and can be used for doing work if desired. Figure 5 shows a bi-directional apparatus 30 according to an embodiment of the invention where tanks are located on either end of the siphon such that entrained air under pressure can be collected for both directions of flow. In many embodiments of the invention the flow of air is used to drive a turbine. The turbine can be placed in fluid communication with gas inlet 40 such that the suction of air as it is being entrained in the fluid flow will suck air through the turbine and drive it. This is advantageous as turbines act particularly effectively when their output is at a low pressure. In an embodiment of Figure 5 a turbine can be placed between tank 70 and air input 40. Thus the turbine is driven by the difference in pressure P2-P1. The power generated by a turbine depends on the difference in pressure between the input and output and the flow of gas. In effect, the power generated is proportional to TM'(1-(P1/P2)0-286), where T is the ambient temperature of the air at the input, and M' is the mass flow rate per second (dependent on the number of air molecules/s). The turbulent losses increase as the ratio of volume (not mass) air flow rate to water volume flow rate, q/Q, at the point of injection increases (namely the venturi in the siphon). This has various implications. The denser the entrained air, the better. Hence, the colder the entrained air, the better. Air cools as it expands through the turbine. There are therefore advantages to be gained by lagging the transverse air pipe, 75, in Figure 6, to stop the air warming up before it gets into the siphons. The output power is proportional to the ambient temperature, T. Therefore, at the turbine input, it may be profitable to use a solar heater on the input. In systems in which compressed air is collected, although the air will be very cold when it enters the siphon, it will have warmed up to water temperature by the time it is collected, without having significantly decreased the temperature of the water. This is partly because water is much denser than air, so, volume for volume, its thermal capacity is much larger. Furthermore, there will be good heat transfer from the water to the air, due to the aforesaid (unwanted) agitation caused by the entrainment. Clearly, increasing the power output by increasing flow has disadvantages in that you need a bigger turbine to cope with the increase in fluid flow. By increasing the pressure difference a smaller flow can provide the same output and thus increase efficiency. Furthermore, a low amount of gas being entrained in the flow of fluid through the siphon will make the siphon more efficient. The more gas being entrained (i.e. the greater q/Q, see above) in the fluid the more the turbulence, an increase in turbulence decreases the efficiency of the system. Thus, this arrangement is particularly effective. Figure 6 shows a particularly preferential embodiment of the present invention with a plurality of siphons 30 being arranged in parallel with each other. In this embodiment the high pressure tank 70 that collects air that was entrained in the siphons runs along the bottom of all the parallel siphons and collects the air entrained in them at pressure P2. There is a similar tank 75 for collecting the air to be admitted to the gas inlets 40 of the siphons 30. Tank 75 is at a pressure Pl that is lower than atmospheric pressure due to the suction effects of the gas inlets. Thus, turbine 80 operates between P2-P1. As can be seen there is a turbine for every five siphon pipes in this embodiment. Clearly different numbers of turbines for different numbers of siphon pipes can be arranged depending on the capacity of the turbines, the size of siphon pipes, the height difference between the liquid levels and the expected flow rates. In this embodiment there are valves 100 that control the input of air to each turbine and these can be varied to control the flow through the turbine and thus, also the amount of gas entrained in the fluid flow. There will also be one valve at each gas inlet (42, 44 see figure 7) so that the gas flow to each siphon can be controlled in at least as much as it can be turned on and off. There may also be an additional pipe and an additional valve which is a valve for admitting gas to the top half of the siphon. This can be used to admit an air pocket to the top of the siphon and control the flow in this way. It can also be used to admit a lot of air to the siphon and thus stop flow. In this embodiment with many siphons arranged in parallel the amount of flow from one side to the other can be controlled by simply controlling the number of siphons that are operational. This can be effective as it may be advantageous to have 5 siphons driving one particular turbine running at full load with the next five not being operational. This may increase the efficiency of the turbines rather than having them all running at a fraction of their full capacity. In the embodiment of Figure 6 one of the siphons can be arranged to have a continuous flow through it. This siphon can then be used to entrain air from vessel 75 into the water and thereby create suction in vessel 75. This can be used to reprime the other siphons if they have become non-operational for some reason. In such an embodiment a valve allowing air to enter and be entrained in the flow must be extremely carefully controlled as obviously a situation where the suction in this siphon is broken should be avoided as in that case it would be difficult to reprime the other siphons. However, allowing some air to be entrained in the siphon would be advantageous as then even when the turbines are not running the water can still be being aerated. This embodiment needs a further valve (not shown) connecting the tank 75 to the gas inlet, thus the air being entrained into the flow can be taken from the tank 75 when repriming is desired and not when it is desired to leave the other siphons non-operational. The embodiment of Figure 6 may be used to control tidal flow to and from a tidal lagoon formed by trapping water behind a barrier. The operation of such a system is set out below. At low tide, the lagoon will have emptied through the pipes, and a shipping gate (not shown) can be opened to complete the equalisation of the two water levels. Then the gate can be closed and flow from all but the special priming pipe can be cut off by admitting air to the top loops of the siphons. In one embodiment there are 40 siphons and flow from 39 of them is cut. When the tide downstream has risen enough for there to be a Im water level difference between the lagoon and the seaward side of the barrier, the air can be evacuated from the vessel 75 by gradually opening a valve connecting the vessel 75 to the special priming siphon. The combination of venturi suction plus siphon suction from this special pipe will draw air out of the other pipes. When this operation is complete, water will flow through all the pipes. The system will now deliver air power both suction and compressed. Part of the compressed air can, if needed, be bled off. This air power can be used to drive a turbine. Furthermore, the air being entrained in the pipes will improve the water quality. Having a single priming pipe running all the time will not have a large effect on water levels but will increase water quality. Furthermore, some of the compressed air could be used to operate the ship gate or indeed any other pneumatic machinery in the vicinity. The siphons are now left running so that the level in the lagoon can approach that on the seaward side. At high tide the shipping gate can be opened to equalise the levels. As the tide beings to fall, the shipping gate can be closed, air admitted to the top of the 39 pipes and the process begun again in the other direction. The arrangement of the gas inlet 40 and the Venturis are shown as 90 in Figure 6 a section through 90 being shown in the Figure 7. Figure 7 schematically shows the gas inlet 90 of Figure 6. As can be seen there are two inlet pipes 42 and 44 leading into the venturi 50 and there are several outlet slots 45 allowing gas to escape into the venturi from respective pipes 42 and 44. The reason for the two inlet pipes and the two sets of outlets 45 are the fact that this apparatus is bi-directional and flow can occur in either direction depending on which side of the barrier the higher liquid level is. This makes it suitable for tidal flows. When the direction of flow is in the direction of arrow A then gas is sucked in through pipe 44 and entrained into the fluid flow. Pipe 42 holds some water which rises up the pipe due to the low pressure in the vessel 75. It is for this reason that the pipes are arranged to extend up into the low pressure tank 75. Each pipe may have a valve on it for opening and closing the flow, although in some embodiments this valve will not be present. This valve is not shown in this diagram. The rationale behind the use of the slots which can be seen in Figure 7 is as follows. The suction developed by the venturi is maximum where the venturi throat cross section is least, namely opposite the humps in the ramped wedges carrying the slots, and gets progressively less thereafter. Hence, the starting point of the slots determines the maximum suction that can be developed by that particular slot. In some embodiments the pipes have a diameter of about 2m and there will therefore be a hydrostatic pressure difference between the top and the bottom of the pipe. By staggering the leading edges of the slots according to vertical location (not shown), the venturi system can be designed to develop a bigger suction at the bottom of the pipe, where the hydrostatic pressure is highest and air entrainment is hardest, than at the top, where the hydrostatic pressure is least, and additional venturi suction is least needed. In this way, first order compensation for the hydrostatic pressure difference in the pipe can be obtained. Secondly, when air is admitted, it will first start being entrained just behind the leading edges of slots, because that is where the suction is greatest. The admission of this air will, however, deprive the water of some of its flow cross section, increasing its speed to that comparable with what it had opposite the hump. This, combined with the suction suppressing effect of admitting the air described above, should have the effect of increasing the suction further along the slot, encouraging more air to be admitted there, and so on. Figures 8a and Figure 8b show an air intake and centrifuge according to an embodiment of the invention. In many situations these siphons are used at the seaside. In such cases salt water spray may end up in the air being drawn into the siphon and this can cause damage to the turbine. Thus, in preferred embodiments there is an air intake centrifuge arranged in the inlet pipe to centrifuge the air and thereby throw any impurities such as salt water to the outside. There is a drainpipe 155 which is blocked at one end such that the water and air being thrown into this pipe loses its speed and the water then drains down and out of the centrifuge. This can be particularly useful in situations where the entrained air is collected in a vessel 70 at high pressure and used as the air inputs of the turbine. In such embodiments air may be quite badly contaminated with sea spray such that the use of a centrifuge is important to ensure a long life of the turbines. In some embodiments the siphon, or plurality of siphons have a control box 162 (see Figure 9) associated with them. This has a microprocessor within in it (in some embodiments within a phone box 167). The signals from the sensors arrive at the microprocessor card within this box and are processed, the processed signals are then sent from the phone box 167, where in many embodiments, a mobile phone transmits the information to a central control centre 172 remote from the siphons. The central control centre 172 receives the signals from the mobile phone at phone box 175. These signals are then logged 170 and a personal computer 180 processes the received signals and any user inputs and generates command signals. Figure 9 shows the central control system 172 and the control box 162, which communicate with each other using signals sent via a mobile phone or in some circumstances via dedicated lines. Eight analogue channels transmit signals from sensors located on the flow control apparatus to the control box 162. These are converted by an analogue to digital converter 160, and are then sent via phone box 167 to the central control system 172, where they are received by phone box 175. Phone box 175 sends the signals to a data log file 170 which can be accessed a PC 180. The PC 180 is operable to both analyse the signals sent from the sensors and to generate commands, sometimes in response to these signals and sometimes in response to user inputs. These commands are sent via the phone box 175 to control box 162, where they are received, converted to analogue form by a digital to analogue converter 165, and sent via 8 analogue channels to control the opening and closing of the various valves associated with the flow control apparatus. The PC can thus be used to both control the operation of the remote site and to report its status. Figure 10 shows a siphon 30 and valves in greater detail. In this embodiment turbine 80 is attached via valve 60 to the siphon. Valve 60 can be used to shut off the air being entrained in the siphon. Thus, in this embodiment air is sucked directly from the atmosphere into the turbine, it passes through the turbine into the siphon where it is entrained in the fluid flow. The apparatus has a further valve 110 which is operable to admit air to the top of the siphon. This can be used to allow a certain amount of air to be present in the top of the siphon thereby limiting the flow. It can also be opened completely to allow air to be sucked in and to break the suction in the siphon and thereby stop flow. This system has sensors S3 and S4 operable to detect the pressure at the inlet and outlet of the siphon respectively. There is also a sensor Sl to detect the fluid flow, a sensor S6 to detect the amount of air at the top of the siphon, and detector S2 to detect the amount of air entrained into the siphon. All of these sensor readings can be used in a control system to control the various valves and thereby control the operation of the fluid flow. This can be used to control the difference in levels 10 and 20 and also to control the amount of air being entrained which can control the amount of power generated by the turbine. In addition to the valves and sensors already mentioned, there are additional valves 120 and 130 which are in a side conduit 140. There is also a pump 150 in this conduit. This pump is driven by the fluid flow in the conduit 140 when valve 130 is open. It thus exerts a suction effect at the top of the siphon when valve 120 is open and can be used to initiate flow, or in other words reprime the siphon after the flow has been interrupted by opening valve 110. The embodiment of Figure 10 is one which has been designed to maintain levels in drainage channels in flat drained marshland, such as in the fens. In such marshlands water level differences between sluice gates are typically 1 to 2 m. This embodiment of the invention can be used not only to control water levels but also to both generate electrical power and aerate the water, thereby improving its quality. At the same time, the apparatus lends itself to automation, allowing many sluices to be managed and monitored remotely from a central control system such as that shown in Figure 9. The general principles are illustrated in Figure 10, in which the traditional sluice gate, or penstock, has been replaced by a siphon 30. The diameter of the pipe used for the siphon is determined by the maximum flow rate which is needed between the upper and lower water levels. When the siphon is primed, the water flows. When air is let into the top of the siphon through valve 110, the flow ceases. If air is injected into the upper part of the siphon through valve 120, partly under the influence of the reduced hydrostatic pressure at the top of the siphon, and partly under the influence of the acceleration of the flow due to the presence of a venturi, the resistance to the water flow increases as the air is entrained within it and its rate is reduced. Therefore, by adjusting the volume of air admitted per second, the flow rate of the water can be controlled. Alternatively, the flow rate can be controlled by admitting more or less air to the top of the siphon via valve 110. The flow rate depends on the relative fractions of the cross section of the top of the siphon which are occupied by air and water. In this embodiment there are 3 valves controlling the system. Valve 60 controls the amount of air being admitted to the system and being entrained in the water. Valve 110 is used to admit air to the top of the siphon and thus, restrict the flow in this way and if enough air is admitted to break the flow. Valve 120 is used to prime the system following an interruption in flow and can be connected to some sort of suction means, such as a vacuum pump. The system can be controlled remotely using a PC and mobile phone technology. In Figure 10, the valve 110 lets air into the siphon, killing the flow. Valve 120 allows air to be pumped out of the siphon, priming it and enabling the flow. Valve 60 lets air into the venturi via the turbine, thereby generating more or less electricity, and allowing the flow of less or more water, over a narrow range. Valves 120 and 110 can be used in conjunction to control the water level in the top of the siphon, and thereby also control the flow, over a wider range, in such a case there has to be active valve management as the siphon is in an unstable mode of operation. This will require a local microprocessor, which is needed anyway to encode the status and command signals, to take readings from a water level sensor in the top of the siphon and respond accordingly. The inrushing air can be used to drive an air turbine, and generate electricity. Typically, this could be DC. The DC current from several such siphons or penstocks can be routed to a central point, converted to AC, and fed into the mains. The air, carried down in the water to the lower level, will bubble to the surface, but some of the oxygen will dissolve, particularly if, before entering the siphon, the water was nowhere near saturated with this gas. Hence, the quality of the water will be improved. An example of the operation of the system is given below. Assume the penstock is closed. There is no water in the siphon. In Figure 10, valve 110 is closed. The upper water level is being monitored by sensor S3 and the lower water level by sensor S4. The electric valve 130 is opened. Water then flows from the upper to the lower level through conduit 140 and the water driven air suction pump, P. Valve 120 is then opened, allowing air to be pumped out of the top of the siphon. When the sensor S2 registers the arrival of water at this point, valve 120 is closed. The siphon is now full, and water will flow from the higher to the lower level. The flow rate is monitored by sensor Sl. Valve 60 is then opened incrementally, till the flow rate, monitored by sensor S 5, reaches a level compatible with the capacity of the siphon for air entrainment, based on the data from sensors S3, S4 and Sl. Should too much air be admitted, sensor S2 will register air instead of water, valve 60 will be partially closed and valve 120 re-opened till the siphon is correctly re-primed. In the event that there is a need to increase the flow of water, valve 60 can be shut. In addition to supplying power to the grid, the DC output can be used to keep a local battery topped up. This battery will power a local microprocessor, connected to a mobile phone. The phone will be able to dial up a central computer to report the status of the valves and sensors, either automatically or on command, or if there is any change in status. In this way, an operator manning the central computer will be able to monitor the status of many such sluices simultaneously. He will also be able to command the sluices, through the computer, into different states of operation. By this method, the water levels over a very wide area can be managed by just one person.

Example For a 0.44 m diameter pipe, if no air power is being extracted, a drive head of 0.5 m will give a flow of 0.3 m3/s, and a head of 1 m will give a flow of 0.42 mVs.

If 1 kW of air power is being extracted, using a head of 1.2 m, the water flow rate will be 0.28 m3/s. It should be noted that these figures are predictions as opposed to experimental results.

Although a particular embodiment has been described herein, it will be appreciated that the invention is not limited thereto and that many modifications and additions thereto may be made within the scope of the invention. For example, various combinations of the features of the following dependent claims could be made with the features of the independent claims without departing from the scope of the present invention.