glow worm compact 100e hot water problems?

I have hot water problems on my boiler, two days ago hot water stopped working. Found one fault which was domestic hot water micro switch diaphragm holed, so replaced also replaced pressure differential diaphragm at same time. The problem now is i have hot water when the c/h is on and afterwards for about 1hr or so (boiler still warm) but when cold no hot water briefly fires up (about 10s) the flame shuts off but pump still running! any suggestions please