Hi 2 possibilities, most likely is Grunfoss selectric thingy which sounds like a motorised valve. If so it will have 3 pipes to it, one bringing hot water from boiler, usually the centre pipe at the bottom of unit, the other 2 pipes take the water either to the heating coil in tank to heat domestic water or to the radiators, the position of the valve is determined by your room & cylinder thermostats, which are enabled by your programmer[UP1].
it looks like the valve has stuck in central heating position so when prog. calls for hot water the flow is going to rads instead, a quick check is to put on HW only and feel pipes leaving v/v only the one to cylinder should burn you! the one to rads should stay cool! Without test gear & know how there isn't much more to DIY, needs local heating co. regards SPARX
I really wish people who have no idea would not give advise, the Grundfos Selectric thingy is a pump and will either be a UPS15/50 or a UPS15/60 and if this was not working the radiators or the hot water would not get hot. Sounds like based on youe vague description that either your room thermostat or more likly the motorised valve is not functioning correctly. Call out a heating engineer to sort this as you can not identify system components it will be difficult to give a diy solution.
OK H-D in this case there ain't much they can do but if all we do is say "call eggineer" then why bother with DIY forum? sometimes a clue may well find operator error etc. & not require outside help, BW
Take your point sparx, however if you note I reply almost on a daily basis to many different questions and try if possible to offer a diy solution or put forward a piece of equipment that may solve the persons problem. However in this case when equipment is referd to as thingy and key components could not be found I was of the opinion that this person would be better with trade help rather than over a forum.