Thanks for the info guys!
Just another thing to point out, when there is no call for heat but the boiler is powered, the buzzing of the relay on the pcb is accompanied by the 3 port valve operating!!!. Gaz, further to your suggestion, does this make sense? and am I barking up the wrong tree...
I have a 4 1/2 year old Potterton Suprima 50 which does not lock out but sometimes will not fire on demand from the central heating (Hot water demand brings the boiler on though).
I've checked the room thermostat / tank thermostat and all appears fine at that end, however, I've noticed a...