The present invention relates to a teeter control for a wind turbine assembly and in particular, but not exclusively, for a two bladed wind turbine assemblies. Two blade type rotors typically generate higher resulting bending moments on the rotor shaft due to asymmetric mid load. To lower these bending moment loads a configuration of the rotor hub allowing a limited oscillation (or yielding) of the rotor hub around an axis perpendicular to the main rotor axis has been adopted. This is commonly known as the teetered suspension of the rotor hub, and the limited extent teeter movement is shown schematically in Figure 1a. The two rotor blades can be connected together and teeter as a whole as shown schematically in Figure 1 b or each blade can teeter individually, independently of the other blade, as depicted schematically in Figure 1 c. The teetering oscillation movement of the rotor is aerodynamically dampened once the rotor is rotating. Before start up the rotor teeter angle has to be set to a starting position by means of a device, known per se, that can move the rotor hub. This device can also be used to control and possibly dampen the teetering angle. Conventional teeter control devices for driving, adjusting and dampening the teetering movement typically comprise a hydraulic piston connected to the rotor and to the main rotor shaft, or rotating counterpart on the main rotor shaft, whereby a linear movement of the piston drives the angular movement of the rotor. This conventional configuration requires the conventional provision of pivot rotation points at the ends of the hydraulic piston. It is an object of the present invention to provide an improved teeter control device operable to move a rotor hub over a teetering angle range and also control and dampen the teetering movement. According to one aspect of the invention there is provided a teeter control comprising an hydraulic piston movable to and fro within an hydraulic chamber, and sealing means to restrain flow of hydraulic fluid between the piston and chamber, wherein said piston is mounted for rotational movement relative to the chamber about an axis which lies displaced laterally from said chamber, said chamber being in the form of a curved passage having a centre of curvature coincident with said axis of relative rotational movement, and said chamber having a wall surface of substantially uniform cross-sectional dimensions over at least that part of the length of the passage at which said wall surface is engaged by the sealing means. A fluid connection preferably is provided in a wall of the hydraulic chamber for allowing hydraulic fluid to flow into and out from said chamber, either to cause movement of the piston or to respond to movement of the piston. The invention further teaches the provision of a pair of said teeter controls interconnected hydraulically such that fluid flowing out from one teeter control causes a corresponding flow of fluid into the chamber of the other teeter control of the pair. An hydraulic line interconnecting to said teeter controls, or interconnecting an. individual teeter control with an hydraulic fluid reservoir, may be provided with a control valve and or a dampening device in a know manner to ensure the required movement and dampening effect. If a pair of teeter controls of the present invention are provided in a wind turbine assembly, preferably they are arranged to have their respective axes of relative rotation coincident. Typically the two controls are positioned at diametrically opposite positions as considered about said axis of relative rotation, that being also the teeter rotation axis. In a further aspect of the present invention there is provided a wind turbine assembly comprising a teeter control of the present invention. Preferably the or each teeter control is positioned in the assembly with said axis of relative movement thereof coincident with the teeter rotation axis. Further aspects of the present invention will become apparent from the following description, given by way of example only, of one embodiment of the invention in conjunction with Figure 2 which shows schematically a sectional view of a teeter control in accordance with the present invention. Shown in Figure 2 is a curved hydraulic piston device 10 comprising a female or hollow part 11 having an internal bore and a male part 12 having a piston part 13 of a shape complementary to that of the bore of the female part and in sliding engagement therewith. Both the bore of the female part and the piston of the male part are curved in their longitudinal direction, the curvature of both being an arc of a circle and the circles being concentric at a point T which is the teeter rotation axis The piston head 13 is provided with radial seals and guiding rings 5. Reference numeral 2 indicates a cylinder cover with seals. Reference numeral 5 indicates the bottom, of the bore of the hollow or female part 11 and reference numeral 4 indicates the connection port for an hydraulic fluid circuit which is operable to vary the relative angular displacement of the female and male parts 11 , 12. In use the teeter angle control device of the present invention is connected between the rotor or rotors and the main rotor shaft or rotating counterpart on the main rotor shaft. The hollow part of the teeter control device may be attached to the rotor or rotors and the male part of the piston may be fixed to the main rotor shaft or rotating counterpart or vice versa. A particular advantage of this configuration of the teeter angle control device between the rotor or rotors and main rotor shaft or rotating counterpart does not require rotation points as are required at the end of a conventional linear hydraulic piston serving the same purpose. Thus in accordance with the present invention the execution of teeter angle control and dampening can be made very compact. State of the art CNC machines can readily manufacture the curved hollow bore and curved piston counterpart with its high radial pressure seals.