1. A method of deriving reference prediction mode values for encoding or decoding of a prediction mode related to a current coding unit, the method comprising: deriving first and second reference prediction mode values from respective prediction modes of at least two neighbouring coding units of the current coding unit, the first and second reference prediction modes being different from one another; and deriving a third reference prediction mode value from the first and second reference prediction mode values, the third reference prediction mode being different from each of said first and second reference prediction mode values; wherein deriving the first and second and third reference prediction mode values comprises checking whether the respective prediction modes of said at least two neighbouring coding units are the same or different and, if they have the same mode and that mode is a planar prediction mode or a DC prediction mode, setting the first, second and third prediction mode values to the planar prediction mode, a DC prediction mode and a vertical prediction mode respectively, and wherein the first, second and third reference prediction mode values are usable for the encoding or decoding of the prediction mode value to be encoded or decoded.