I have a baxi 105e boiler that was fitted to the house when I moved in but over the past year the central heating temperature has dropped. Now only the rads near the boiler get warm. I had a plumber in who checked the pump all OK, checked some filter all OK and then said the diverter valve was duff and needed changing. He did this but it has made no difference all the radiators are fully opened up but as soon as the boiler fires the internal bypass pipework gets red hot and in a minute the boiler shuts down. It looks to be just pumping the central heating around the boiler like the bypass is open all the time. Any ideas?