I have a Potterton Puma 80 combi boiler about 4 years old. If I light the pilot, then run the hot water, it fires up. But the flow rate stays low. Then after about a minute the pilot goes out. After that if I run hot water, it is still low flow rate but very hot. Similarly, if I light the pilot again, then I run central heating, the boiler fires up but the flow and returns stay cold. Then after a minute the pilot light goes out. The low pressure light never comes on and I think that the pressure level is adequate. The fan works and the steam release valve (on top of boiler) is loose. I think that the boiler overheats and pilot cuts out because it is not allowing any water to flow whether CH or HW. But I don't know how to isolate the fault. Any advice would be appreciated, thanks.