1. A method of predicting motion vectors associated with a block of pixels of a picture to be included in a data stream for differential motion vector coding of a video signal, said method comprising the steps of a) organising a set of reference pictures in to a pair of lists and according to each reference picture within said lists at least one reference index, b) associating with selected ones of said blocks in said video signal at least one motion vector that references a respective one of said lists, with each vector associated with a selected one of the blocks referencing a different list, each of which vectors defines dispositon of said block relative to a reference picture in the respective one of said lists, and c) computing a predicted value for each of said vectors from vectors of adjacent blocks referencing the same list as the vector being computed, whereby, the prediction of a motion vector that selects a reference picture using a certain list of pictures is not dependent upon the motion vectors whose reference pictures are selected using the other list of reference pictures.