Pilot light keeps going out with my Potterton Puma 80 boiler?

Hi,
We have a Potterton Puma 80 boiler that lives in an outdoor shed attached to our house. Our problem is that every time we run hot water for a bath or shower, the pilot light goes out, which means we also have no central heating. A plumber has looked at it and serviced the boiler, but we still have the problem. He now reckons that the pump needs changing and some sort of inner valve, which costs a fair bit. Does that seem right? Any other thoughts as to what the problem could be? Thanks. We also had a time when our heating wasn't working at full capacity (over the deep snow period) and our plumber reckoned we needed to flush the system, but suddenly it started working again so we are hoping not to have to do that at the moment as we don't have much spare money for all this. Any advice welcome.