Thanks for the reply by the secondary plate heat exchanger I assume you mean the small unit at the bottom of the boiler held in place by two screws?If so I have removed it and flushed it through there was some sludge in the bottom but not a lot I wasn't aware there was a primary and secondary...
when I turn on my hot water the boiler ignites but then the flame go's out after approx 15 secs then re-ignites and does so until I turn off the hot tap I have changed the thermistor but problem continues?