1. A computer-implemented method of global motion estimation between frames of a motion-compensated inter-frame encoded video sequence, the method comprising, for a particular frame of the inter-frame encoded video sequence: a) determining, using one or more computer processing systems, a motion estimation representative of the global motion between the particular frame and a first preceding or first succeeding frame of the inter-frame encoded video sequence, on the basis of motion vectors directly between the particular frame and the first preceding or first succeeding frame along a first route between the particular frame and the first preceding or first succeeding frame; b) determining, using the one or more computer processing systems, one or more further motion estimations representative of the global motion between the particular frame and the first preceding or first succeeding frame at least partially on the basis of motion vectors between the particular frame and one or more preceding or succeeding other frames along one or more second routes between the particular frame and the first preceding or first succeeding frame, wherein the one or more second routes are different from the first route; and c) selecting, using the one or more computer processing systems, one of the motion estimations which meets at least one predetermined criterion as being representative of the global motion of the particular frame; d) storing or outputting said selected motion estimation for use in registering an image of said particular frame with an image of said first preceding or first succeeding frame.