"I hope someone can help me out here!
My heating (baxi back boiler) was working this morning, but isn't now.
But the hot water's ok so boiler, thermostat, pilot, pump etc are working?
Many thanks to all contributors for correctly diagnosing the fault (3 port valve) and for a load of background info and references that helped enormously in fixing it.
The valve was an elderly one piece Gyr and Landis, long out of print.
My friendly local - ish shop (plug for Bookers of Wickford) suggested a Danfoss 2 part unit as a plug in replacement which went in perfectly and worked first time. The price made me eyes water a bit, though. But he would take it back if I didn't think it would fit.
Incidentally one reason for panic is that all sensible shops like tools, bearings, 3 port valves are miles away, I use a motorbike and ice and snow were forecast - could visualise me and mog being Cut Off, socked in, freezing, last diary entry "Great God, this is a dreadful place, have just ate the cat and out of lager..."
The wiring colour code wasn't identical but the background info sorted that.
Draining down wasn't too bad as all my rads are top down fed so only some piping needed emptying.
Fired it all up again and worked a treat, moggie and me are snug again.
So thanks again for all the expertise freely given, I'll get you all a cyberpint