I have just realised I didn't post the fix for this. I'm really sorry. The AFM seemed to be working but we had one we ordered for a Nissan Primera diesel that was never fitted. The unit was the same (Bosch) so we tried it and the problem was solved. It's interesting that the signals being generated by the old sensor were around what I would expect but it was causing such an extreme loss of power. Once again, I apologise for not posting this at the time but I was in shock at having repaired something straight away!!