The system is a Honeywell Y plan comprising of Switchmaster 600 (multi function programmer), a Thorn Olympic 38/50 boiler, Honeywell diverting valve, Honeywell room stat. The heating and hot water are both set to constant but the programmer seems to have a mind of its own, it comes on and off when it feels like it. At one point for a few days running it seemed to follow some kind of pattern, it would behave perfectly all night on low temp setting and work fine all day until the evenings around 6pm and then nothing, dead as a door nail, until sometime after midnight when we've all turned in it would come back to life, almost as if the programmer was on timed. As I said that was at one point but as things stand at the moment we could loose the heating and hot water at any time for one or two hours and then it would suddenly decide to start working. My guess is the Switchmaster has blown a circuit but any other ideas from you guys would be appreciated.