Hmmm. As usual, a lot of sense in what you have to say. I have just used the hot water and it's much hotter than it should be which concurs with what you said about the boiler modulating. Getting to everything to play with a multimeter is quite difficult because it's all at the back of a big airing cupboard, including the programmer which the electrician promised to move but of course never did. That's why I can't hear if the microswitch is operating or not. I'm tempted to take a gamble and replace the valve. It's not mega-bucks after all and at the very least it would eliminate a suspect. Is it much of a job ?