I have a 10 year old Potterton Puma boiler. The heating is coming on as usual but it cuts out after about 30 seconds and then starts again after another minute. It repeats this pattern all day. Consequently it takes at least an hour for any heating to get through to the radiators. When they finally get hot, the first four radiators on the circuit work well. But the next five radiators won't work at all so I have no heating at the front of the house, upstairs or downstairs. I have a British Gas Homecare agreement and they have changed the pump and diverter valve and now say that the heat exchanger is clogged up and the system needs powerflushing. Apparently I am not covered for either of these two jobs under the Homecare agreement. I have had a plumber manually flush the radiators which are not working and there is no blockage there and the general consensus is that the problem is with the boiler which is not pumping enough to get the heat around to all the radiators. All British Gas want to do is to sell me a new boiler. I just want the boiler to work without cutting out and for all the radiators to work.