Hi, Having trouble with boiler, Ideal Classic NF 230 Open Vented System. Boiler fires up all good and runs for 10-20mins then flame cuts out, however fan keeps running and the sparker goes constantly as if trying to relight. There's no smell of gas. The overheat reset button doesn't pop, the overheat stat has been replaced anyway but same story. The system was recently drained but has been bled and in the time the boiler is on the rads start to heat so seems the water is circulating. Tried turning thermostat knob up and down and at very low setting all seems to work normal but turn it up at all and the problem appears. Any ideas?