[RaelG]

Yes I came across a wiring diagram that suggested that the wire goes via the immobiliser too, at least in Europe anyway. So yeah maybe there's something in that. I have no idea if the immobiliser is working or not. Maybe I need to get into the ECU and check there but to be honest I have no idea what I'm looking at, I'm a long way from an auto-electrician in terms of skill!

I've only driven the car twice since buying but I did notice that about the rev counter. Maybe I'll take it out again and keep a closer eye. I think you might be right though, a decent auto electrician might be the next step. Only problem is...who?!