Just bought a house with a Baxi Solo 2 boiler. Boiler makes a noise like a steam train when it's running. If the room stat is calling for heating and the (Grundfos) pump is running, but I turn off the boiler, water circulates quietly. Had a plumber look at it, and he thinks it's full of sludge, and moaned about it having no drain tap. He suggested putting Sentinel sludge remover in for a few weeks. Looking at the boiler, it appears that replacing the heat exchanger would be quite straightforward. Are they still avaiable? How good is the Sentinel anti-sludge stuff? Can it really unblock a boiler that's had no TLC for years? Any other suggestions?