recently had a problem with my heating, rads were cold upstairs , so i went through the procedure of bleeding etc. this worked perfectly to begin with, after about half an hour however the boiler refused to to light even with the thermostats on the boiler and in the room turned up full( no thermostats on rads). the boiler is an old baxi back boiler unit, with gravity hot water and pumped heating ( the pump seems to work okay although it only operates when the h/w is turned on on the programmer).any advice would be greatly appreciated.