1. A method of determining a blocking artefact measure relating to blocking artefacts in a digital image comprising a plurality of pixels having a value, the method comprising: for each of a plurality of pixels determining a vertical gradient measure in dependence upon the values of said pixel and horizontally and vertically neighbouring pixels, in which the vertical gradient measure is equal to the greater of (a) the difference between an average of the values of said pixel and three pixels to a first side of said pixel and an average of the values of four pixels positioned vertically to said pixel, and (b) the difference between an average of the values of said pixel and three pixels to a second side of said pixel; and an average of the values of four pixels positioned vertically to said pixel; comparing said vertical gradient measure with a vertical gradient threshold and defining said pixel as a potential horizontal boundary in response to the vertical gradient measure exceeding the vertical gradient threshold; determining a horizontal gradient measure in dependence upon the values of said pixel and horizontally and vertically pixels; comparing said horizontal gradient measure with a horizontal gradient threshold and defining said pixel as a potential vertical boundary in response to the horizontal gradient measure exceeding the horizontal gradient threshold; and determining said blocking artefact measure in dependence upon the vertical gradient measure of pixels defined as the potential horizontal boundary and upon the horizontal gradient measure of pixels defined as the potential vertical boundary.