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Further developments since my original post. We have since received a letter thanking our 'guest' for confirmaing his address. The letter goes on to ask why the Halifax hold a correspondence address for him and why he has tenants in the property?
My wife went into the local Halifax branch, explained what is happening and what the Halifax can do to help. She was seen by a member of staff who wasn't much help. She went to speak to a manager and returned after approx 2 mins. She said all she could do was to take the documents from my wife so they can be copied and handed back. My wife didn't have them with her and as these are the only prrof we have of what is happening to us, we're holding onto them.
It gets worse one of our 'guests' has defaulted on a loan, and failed to make recent payments.
Why us? How the heck did these people get our address from? We are so careful when destroying all letters, envelopes etc with our names, addresses etc.