Urgent! Vaillant boiler works hot water not central heating?

Make sure the pressure is correct on the boiler gauge. It should read about 1.0 to 1.5 bar when cold. Repressurise with the filling loop. It may just be that the CH controls aren't set right. Start with the CH control which should be on the face of the boiler case. It is often shown as a radiator icon. Then make sure your room thermostat is set to call for heat i.e. 21 C. If the boiler doesn't fire then there is probably a timer/programmer issue. If you have a timer on the boiler set it to 'constant' or whatever implies that. If the boiler still does not fire, yet works on HW there is a fault such as sticking diverter valve (a job for a CORGI man). If you go to www.vaillant.co.uk you can get to manuals etc after you key in the boiler name. This may help in further diagnosis.