Why does my boiler cut out when water heater/ central heating kicks on?

I have a potterton kingfisher cf 55 boiler, which worked well until recently. Now I can have the system on, with the pilot light lit all day and there is not a problem. When I then turn on the central heating and/ or the hot water, the boiler fires up but then cuts out after a couple of minutes and the pilot light goes out. Does anyone know why this is happening please?