Thanks for the advice. It is the CH temperature sensor that is faulty - I've measured the resistance as the water heats up and when it gets to a certain temperature it goes virtually open-circuit (and then recovers as the temp falls).
Replacing it looks straightforward enough but I assume...
My Worcester 240 combi boiler has just started cycling on/off more quickly than normal when the central heating is on. It fires up for just a few 10's of secs (occasionally minutes) at a time. Sometimes the rads get as hot as normal but generally they stay cool. I can hear a relay switching...
