1. A method of reducing the number of motion vectors associated with a digital television image, the method comprising the steps of: deriving for each block of pixels within said television image a plurality of motion vectors representing the most common motion vectors of pixels in said block, each of said plurality of motion vectors having associated with it an indication of the accuracy thereof; associating with said plurality of motion vectors, a further motion vector for the same position from the previous field and a further motion vector for the same position from the next field, each of said further motion vectors having associated with it an indication of the accuracy thereof; and selecting a smaller plurality of motion vectors from said plurality of motion vectors and said further motion vectors in dependence on said indications.