tank of hot water but no central heating?

Just to add to what Tony S has said. I had the boiler repairers out twice and each time it was the synchron valve that was at fault and we froze in the house while we waited for the repair man to come and we paid a big bill. Another heating engineer showed me the easy way out. In your situation just pull the lever (there will be resistance) on the top (or side) of the motorised valve and hook it onto the nib at the side of the slot. This will hold the valve open and you will have hot water in your radiators. You can release it when you want to close the valve and you might just find that this was all that was needed to unjam the valve. If not, at least you will be warm until the engineer arrives.

Colin
November 2008

Just noticed you said you had a diverter valve.Put the clock rocker to water and heating and then look on the side of the motorised valve.There should be a metal lever.Push it and see if there is a resistance.If there is it will be the synchron motor inside that has shot.Make sure any Room stat is calling for heat first.You can buy synchron motors for about £6 from your local plumbing and heating suppliers.

Tony S
September 2008

If its an old Randall 102 Pete,put the rocker on the side to water and heating and then hold a screwdriver to the pumphead.If it is buzzing the pump has stuck and needs a clout to free it.If there is no power to the pump the clock could be naff,but they are easy to change.Grub screw under the bottom,the black cover comes away,then pull the whole clock upwards and the module lifts away from the main body.Just buy a replacement module slide it on,reset the times and rescrew the bottom cover on.Easy peasy.

Tony S
September 2008