1. A spatial decorrelation module configured to perform spatial decorrelation on a block of data values as part of a data compression system, wherein the data values in the block are arranged into a two dimensional array, and wherein the spatial decorrelation module comprises a plurality of parallel processing pipelines each comprising a first stage and a second stage, wherein the first stages of the processing pipelines are configured to implement first spatial decorrelation in a first dimension, and wherein the second stages of the processing pipelines comprise respective storage units and are configured to implement spatial decorrelation in a second dimension, wherein the first stage of each of the processing pipelines is configured to, in each of a plurality of iterations: (a) receive one or more of the data values from a first line of data values from the block; and (b) implement first spatial decorrelation in the first dimension on the received data values to thereby output, to corresponding second stages of the processing pipelines, a first coefficient for a respective one of the data values of the first line; and wherein the second stage of each of the processing pipelines is configured to, in each of said plurality of iterations: (a) selectively store the first coefficients for the first line in the respective storage units; and (b) implement second spatial decorrelation in the second dimension on first coefficients for a second line for which the first spatial decorrelation was performed in a previous iteration, to thereby output a second coefficient for a respective one of the data values of the second line, wherein the second coefficients are spatially decorrelated data values, wherein the second stages of the processing pipelines are configured to selectively store the first coefficients for the first line in the respective storage units if the first coefficients for the first line will be used for the second spatial decorrelation for another line of the block on a subsequent iteration, and wherein the first stages of the processing pipelines are configured to receive the lines of data values in a particular order such that if the first coefficients for a particular line will be used for the second spatial decorrelation for another line then the data values of the particular line are received at the first stages in an earlier iteration to that in which the data values of the other line are received at the first stages.