Well, tried a new cap, only to find that it wouln't tighten onto the expansion tank, on inspection i found the thread on the tank had partially gone. I bought a new expansion tank from main dealer (none in scrap yards) cost £38. fitted it with the new cap, and PROBLEM SOLVED, I think the old cap tightened up then slipped the theads when it was under pressure.
WOW and i was thinking head gaskets and everything,. Not cheap for what it was but it fixed it for me.
THANKS FOR ALL YOUR HELP ...