As I mentioned before, on the odd occasion as the engine was cranking over it would almost stop, struggle to get over compression then go on again. Obviously been using the starter quite a lot since its been here and it also developed a squeak. Took it out and lubricated the pinion and front bearing as far as possible, now magically the starting problem seems to have disappeared. Have since fitted a new starter and will be giving it a good test before the customer has it back. In retrospect I suppose it should have been one of my first investigations instead of the last. Will have to invest in an ampclamp