2002 Jaguar XJ8 Restricted Performance/ TC not available

Hi
I've just purchased an 02 XJ8 sport. When I turn the ignition on a message flashes saying that TC is not available, it then goes. Also, every so often a message will flash up (whilst driving) saying Restricted Performance. It comes and goes, sometimes staying off for 100's of miles and at other times its more frequent. When it does become more frequent the engine management light illuminates. Performance doesnt seem to be an issue and the engine isnt missing a beat?
I've got no problem taking it to a garage for a sensor replacement but in my experience it can be hit and miss finding a garage that will diagnose correctly first time, and I'll end up paying for parts that arent required until they stumble accross the fault.
I am an industrial fitter/electrician so if there are any common faults that could be causing this like dirty sensors, bad connections etc I'd rather take a look myself first.
Any ideas?
Thanks
Jamie