It turned out to be the main earth from the negative term on the battery to the chassis/engine.
I also got some other nasty symptoms like the STOP light flickering on and off and also the cooling fans cutting in very early or not turning off.
I think my fuel ecconomy and performance have both improved since replacing the cable too. I think it must have become slightly broken inside since I removed the battery a few times.
Just incase anyone has similar problems I thought i'd just throw that one in there
________________________________________GTI-6 & Rallye
Rallye gets sideways in the wet at Snetterton