[QUOTE 1809302, member: 45"]The trouble is, I was out in the car yesterday and it won't go over 4000 revs. I'm assuming that it's in some kind of safe mode because of the warning light? Without going to the garage can I pin this down to one or two possibilities, or could it be anything?[/quote]
This is probably totally useless info, but I'm bored so I'm going to type it out anyway:
I had a Vauxhall Vectra Turbo-Diesel a few years ago which would do just that (go into 'limp home' mode on a regular basis). Stopping the engine and re-starting would always cure it. Numerous visits to the garage failed to identify a problem until one day when it happened I had a good look under the bonnet and saw that a 4" diameter hose had come adrift. I think it was something to do with the turbo (I ain't a car mechanic!). I re-fitted the hose with the jubilee clip that was on it and the problem never happened again. My guess is that it had been loose for a long while and was part dropping off from time to time, causing the electronics to think there was a problem and putting the car in 'safe' mode.
So, FWIW, my sugestion would be to poke / prod / jiggle anything you can see under the bonnet and check for loose hoses / pipes / electrical connections.