Purchased new in 1997, Potterton changed heat exchanger in 1999 to cure steam train, rattling which seemed to work. Noises have gradually got worse again until now where its too dangerous to use in my estimation. Any reasons for this, nearly every plumber tells me of the faults with this boiler, do I have any come-back on Potterton/Baxi? They chooses to ignore completely what I say. Have heard it was poorly designed in terms of the heat exchanger, circuit board etc.