Quick question on behalf of my father. He has been trying to get an old Baxi Bermuda 552 boiler unit to work, after a recent stoppage. The boiler is meant to have a 20mm pilot light, which it does. The pilot stays on in its own. However, trying to use the boiler is more problematic. The burner flame is about 1.5-2" high, yellow, and appears to be slightly flickery. After being on for ten or so minutes, it cuts out - and so does the pilot light. He has already changed the thermocouple. Any ideas or suggestions?