Right, slight improvement.
I've turned the dial for the temperature of the water on the front of the boiler - from about 3/4 to under half. The boiler now runs for about 30 seconds at a time, and there is no more banging. It then switches off with a little click, the boiler still reads "c" - which indicates it's working (no error, and not in standby), and the ignitor light flashes. It waits there for a few minutes, and then kicks in again - lights up, pumps for about 30 seconds, very quiet, and then clicks off. If it was overheating, shouldn't i get an error code on the front?
I have central heating, and i have hot water. The mechanical valve is definately working, i've tested it when unscrewed from the system. However, whatever setting i get, the pipes going in either direction seem to heat up just as much as each other, both getting very hot to the touch?? I've tried manually setting the valve, same result.
Finally, how do i set this "auto bypass valve" - the white vertical knob near my CH pump?
Cheers, Patrick