1. A machine-implemented method for motion estimation in a sequence of video images, comprising: subdividing each field or frame of a sequence of video images into a plurality of blocks; assigning to each block in each video field or frame a set of candidate motion vectors; for each block in a current video field or frame, selecting a vector that produces a best match to a block in a previous video field or frame, from the set of candidate motion vectors assigned to that block; and forming a motion vector field for the current video field or frame using the selected vector for each block, wherein the set of candidate motion vectors assigned to a block include one or more candidate vectors derived from an external source vector field, and wherein each candidate motion vector from an external source is assigned a weighting value, that biases towards or away from the selection of that candidate motion vector.