Many thanks for such a quick reply. It is a sealed pressured system. As far as I know there are two motorised valves on the pipework in airing cupboard. There is no thermostat as such on the cylinder, but you can plug it in to get hot water if needed in emergency. Sorry to be vague but am not...
We have a Gloworm Ultimate boiler (reasonably old). It works fine but with one problem. When the central heating is on it fires up and gets all the rads hot then the rads cool and if you turn the thermostat up it clicks as though it is about to fire up...but doesn't. THe only way to get it to...