1. A method of deriving reference intra prediction mode values for encoding or decoding a mode value representing an intra prediction mode for a current unit to be encoded or decoded, the method comprising: deriving first, second and third reference intra prediction mode values to encode or decode the mode value for the current unit from respective intra prediction modes of at least two neighbouring units of the current unit, wherein the first, second and third reference intra prediction mode values are different from one another, wherein the number of the reference intra prediction mode values, to encode or decode the mode value for the current unit, is three, wherein, deriving the first, second and third reference intra prediction mode values comprises checking whether the respective intra prediction modes of said at least two neighbouring units are the same or different and, if the respective intra prediction modes are the same and are either a planar prediction mode or a DC prediction mode, setting the first reference intra prediction mode value to a mode value corresponding to the planar prediction mode, setting the second reference intra prediction mode value to a mode value corresponding to the DC prediction mode and setting the third reference intra prediction mode value to a mode value corresponding to a vertical prediction mode, and wherein the first, second or third reference intra prediction mode value is usable for the encoding or decoding of the mode value to be encoded or decoded.