1. A method of processing a digital video signal to derive motion vectors representing motion between successive fields or frames of the video signal, comprising the steps of: calculating a difference between the contents of each of a plurality of first regions in a first field or frame and the contents of a corresponding one of a plurality of second regions in a succeeding field or frame, each said first region comprising a block of pixels and each said second region comprising a plurality of contiguous blocks of pixels, said block of said first region and one of said blocks of said second region representing a common block; producing for each said first region in the first field or frame a first correlation surface representing said difference; weighting the first correlation surfaces of a respective one of said first regions and of a plurality of first regions adjoining said respective first region, the first correlation surface of said respective first region having the greatest weighting to accentuate features of the first correlation surface of said respective first region relative to those of the adjoining first regions, all the points within a given correlation surface being weighted equally; summing the weighted first correlation surfaces to generate a grown correlation surface; and deriving from each grown correlation surface a motion vector representing the motion of the content of said common block as a function of a minimum value within the grown correlation surface.