Hi wasn't in the pub!!:D:D:D
The overflowing has stopped now and hasn't happened for at least 24 hrs. Yes, I checked both stopcocks and they both shut off fine. The expansion tank is now nice and cool.
Thank you all for your kind help.
Sorry no, it started the day before yesterday. Nothing changed plumbing wise, although the water pressure has reduced slightly. Thanks for your response,
My vented hot water tank is overflowing into the expansion tank in the attic, which, as a consequence is overflowing through the overflow pipe to the outside. Only recently had the ballcock in the cold water tank replaced. Hope there is a quick fix for this! Please assume I am a complete idiot...