1. A data processing apparatus for detecting at least one of a set of code words which has been embedded in a water marked image, the apparatus comprising a registration processor operable to register the water marked image and an original version of the image with respect to one another, and a detection processor operable to recover an estimate of the code word from the water marked image by comparing the registered water marked image with the original version of the image, and to detect the code word by correlating each of the code words of the set with the estimated code word, wherein the registration processor includes an image analyser operable to divide a frame of the water marked image into a plurality of image blocks, a distortion vector probability estimator operable to estimate for each of the image blocks of the water marked image an observed probability of possible distortion vectors representing shifts of the image block with respect to positions of the block in the original version of the image, a forward probability estimator operable for each of the image blocks to generate an estimate of a forward probability distribution of possible distortion vectors within the image block recursively from the observed probability of distortion vectors for the image block and forward probability distribution estimates determined from previous image blocks for a predetermined path through the blocks, a backward probability estimator operable for each of the image blocks to generate an estimate of a backward probability distribution of possible distortion vectors within the image block recursively from the observed probability of distortion vectors for the image block and backward probability distribution estimates determined from subsequent image blocks for the predetermined pattern, and a combining engine operable to combine for each image block the forward probability distribution estimate, the backward probability distribution estimate and the observed probability of distortion vector to form a most likely distortion vector estimate, and to form a map of distortion vectors from the most likely distortion vector estimate for each image block, and an inverse distortion engine operable to apply a reverse of the map of most likely distortion vectors to register the water marked image with the copy of the original image to improve a likelihood of correctly detecting the water mark code word.