Radiators not heating up?

Two radiators are not heating up, but two are. The cold ones remain stone cold. I think it is a potterton combi boiler. The boiler comes on when I switch it on but doesnt seem to do much. Two radiators do heat up eventually but two radiators dont get hot at all. It was recently serviced by British Gas as I have a contract cover witht them. Can you help? Could there be a burst pipe, which could explain why the rooms are so cold and mouldy and damp ridden. Hmmm really cosey! Please help and give me your advice.