Thanks for all your replies, I'd hazard a guess at it being carbon as it's very easily wiped away with a cloth / finger. No idea if this cars been mapped or not. It has a cobra back box section on it though. Any guesses as to why it is running rich?
If it's carbon it's really just too much fuel in the air/fuel mixture, could be a faulty oxygen sensor happened to my brother then the engine light came on a few months later on because it's running rich from the above sensor, so don't worry about that if it pops up.
Are the inside of the exhaust tips very very black? this helps tell it's carbon. If they are clean the inside and see how long it takes for them to get back to that state if it's a week it's running rich, not a definitive test though.