Worcester boiler, heating allright but no hot water!

[justine]

I have a very very very big problem with my combi boiler. The heating is allright but when I want to turn on the hot water the boiler starts and stops starts and stop and it's like that for about three times and it totaly stops. The hot water isn't there, but when my radiatiors are hot the hot water's there. It's ok during the wintertime but what about the summer? uh, there was an engeneer to look at this, he done something, said it's allright and IT WASN'T! It's a bit scarry because when it's trying to start it sound like there was gas gathering there and suddenly ligths on with a strange a bit loud ''PUF'' sound. HELP ME PLEASE!