1. A method of optimising Rate Distortion Optimisation candidate assessment in hardware apparatus, comprising: determining which Rate Distortion Optimisation candidates out of a plurality of candidates are separately processable; determining a processing delay of critical portions of candidate assessment hardware; determining a rule set governing how the separately processable candidates may be ordered for processing; determining an optimised processing order for processing the separately processable candidate assessments, dependent on the processing delay of critical portions of the candidate assessment hardware and the determined rule set; and processing the candidates according to the determined processing order.