Do the Radiators actually heat up while the boiler is running? if not Check the Pump it may not be working
otherwise you would be wise to get an EXPERIENCED and well equiped Service Engineer in, to check the thermostat with a Digital thermometer, there could also be a problem with the burner such as capacitor or just about anything
however in this instance and from your description my cash is on the circulating pump!!
www.elanar.co.uk
November 2009