Just to close this one off, the cap I bought did not fit (thanks ebay) so this morning I took the car back to New Elms. Thirty seconds after arrival it had a new cap fitted! That was clearly the problem, so I think all is well. As it had lost a bit of coolant, Tim is kindly flushing the system and refilling with fresh stuff, just to be on the safe side. I'll pick it up Wed I suspect.
