1. A data processing apparatus configured to receive a down-sampled source block generated from a source frame and to receive a down-sampled reference frame portion generated from a reference frame, said reference frame and said source frame being taken from a sequence of video frames, said data processing apparatus comprising: interpolation circuitry configured to interpolate between pixels of said down-sampled reference frame portion to generate a set of interpolated down-sampled reference frame blocks; cost function calculation circuitry configured to calculate a cost function value indicative of a difference between said down-sampled source block and each of said set of interpolated down-sampled reference frame blocks; minimisation circuitry configured to select an interpolated down-sampled reference frame block which corresponds to a minimum of said cost function value; and estimate motion vector generation circuitry configured to generate an estimate motion vector in dependence on said interpolated down-sampled reference frame block selected by said minimisation circuitry.