1. A method of analysing image data to quantify a block-artefact presence parameter due to block-based processing, the method comprising: deriving a set of pixel values from said image data; spatially filtering the pixel values; evaluating energy within a band of spatial frequencies including spatial frequencies of expected block sizes; evaluating energy outside the band of spatial frequencies including the spatial frequencies of expected block sizes; and, flagging the presence of block artefacts when the energy within the band of spatial frequencies including the spatial frequencies of expected block sizes exceeds the energy outside the band of spatial frequencies including the spatial frequencies of expected block sizes by a threshold value.