1. A method of decoding a bitstream of encoded video by reconstructing significant-coefficient flags for a transform unit, the transform unit comprising a sequence of blocks, the bitstream encoding sets of significant-coefficient flags, each set corresponding to a respective block, the method comprising: for one of the sets of significant-coefficient flags, selecting a context set for use in decoding significant-coefficient flags of that set based on a position within the transform unit of the block corresponding to that set of significant-coefficient flags; and decoding the significant-coefficient flags of that set using the selected context set.