OK, here's the full problem, the boiler keeps short cycling, the flame stays on but dies down, initially I thought as this was affecting the water temp, hot /lukewarm it was related to the water or heat exchanger, however the boiler locks out without water on, seems water just goes cold when running and then hot again due to frequency of short cycling, so far I have changed dhw and ch thermos, I have also changed the flow meter and the Pcb, it's getting costly, I have checked pump, it is very hot, not warm, I mean very hot. I have taken disc off at front of pump, it is turning and vibrating with little noise, radiators get hot initially and as short cycling I'm assuming they cool for approx 30 seconds then boiler kicks in again, I'm at a loss, pipes going in and out of pump both very hot, could pump be working below efficency, pumping water around, however getting so hot that it locks boiler out?