The bank then attempted to start repossession proceedings until Which? intervened, forcing them to hold off until the case was heard by the Ombudsman.
Which? has written to NatWest asking for an explanation for its behaviour and to the Financial Services Authority (FSA) to express its concern that this may not be an isolated case.
Which? chief executive, Peter Vicary-Smith, said: "Which? believes that NatWest’s behaviour is as outrageous as it is baffling.
"The Ombudsman’s ruling could not have been clearer, so for NatWest to appeal and cause this family even more stress without any explanation is, in our view, absolutely disgraceful. We’re concerned that this could be happening to other homeowners around the country."
Marian and Peter Addyman said: "We are delighted that our complaint about NatWest has been upheld. It has caused us and our family great distress over the past seven months – we are very relieved that it is finally over.
"We were very disappointed that it had to go this far – we were always willing to talk to the bank, and wrote letter after letter asking for an explanation. We even offered to pay a year up front! And all we met was a wall of silence.
"We would like to thank our families and friends who have been so supportive, as well as everyone at Which? who have guided us through a minefield of paperwork. Given the outcome of our case, we would very much hope that the bank does not treat anyone else the way that they treated us – we’re just glad it’s over."
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