heating only comes on if hot water on?

we had a new worcester greenstar boiler and system installed by british gas a few years ago. we have a control box and WR1 Drayton Digistat SCR wirless controller in the airing cupboard and a room thermostat downstairs. I have had to replace the batteries in the room thermostat and since then I can only get the heating to come on if the hot water is also on. I have followed the proceedure to re-pair the wr1 with the room thermostat and it all worked ok (green light on on wr1 and room thermostat shows they are communicating) BUT the boiler will not light up and the radiators heat up unless I have the hot water on. Anyone have any ideas ?