1. An image tracking system which determines a position of a target image as the position of the target image varies, the image tracking system comprising: first means for presenting a sequence of images, second means for determining the position of the target image in a previously presented image from the first means, third means for determining movement of the target image between the previously presented image and a subsequently presented image from the first means, and fourth means for producing an output indicating the position of the target image in the subsequently presented image as the position determined by the second means modified by the movement determined by the third means, wherein the third means is arranged to determine movement at a plurality of image elements in a second image portion which is wholly within and smaller than the target image, the third means is arranged to determine translational movement of the target image, and the third means is arranged to solve a set of equations: EQU f.sub.1 (x.sub.i,y.sub.i)=f.sub.2 (X.sub.i,y.sub.i)+.DELTA.x.delta.f.sub.2 (x.sub.i,y.sub.i)/.delta.x+.DELTA.y.delta.f.sub.2 (x.sub.i,y.sub.i)/.delta.y where x.sub.i and y.sub.i are Cartesian coordinates of an ith image element, i is each integer such that 1.ltoreq.i.ltoreq.j and j is an integer greater than one, f.sub.1 and f.sub.2 are functions representing the previously and subsequently presented images and .DELTA.x and .DELTA.y are the Cartesian components of the movement.