Solved! I've just discovered what I was doing wrong. I need to tell GhostScript to use a device of ink_cov not inkcov. (Very confusing naming!) Anyway this now seems to be giving me the actual ink usage so I can save my £136 after all! I just need to make it into a tidier .bat file and along with my spreadsheet it should do exactly what I need. Thanks very much to mwenz for giving me a pointer towards the solution.