heating probs?

I have a Chafetaux Britony combi boiler that works fine when you need hot water. The problem sarts when you switch the heating on. It fires up no problem then after about 45 seconds the flame goes out the pump runs on for about three minutes and it seems as though the heat generated is being pushed up the hot water pipe rather than the heating.
Do I need a new three way valve or do I need a new flow switch