Data processing apparatus comprising: a chunk store configured for containing specimen data chunks, a discrete manifest store, configured for containing at least one manifest that represents at least a part of a data set and that comprises at least one reference to at least one of said specimen data chunks, the processing apparatus being operable to process input data into input data chunks and to identify at least one manifest in the manifest store that includes at least one said reference to a said specimen data chunk corresponding to at least one of said input data chunks.