OK, I have some advice. I ignored Buchanan Clark and Wells who were asking me to pay nearly £500 for a British Gas bill which I never knew I had, dating back to early 2006. They sent a final demand. I called British Gas. Turns out British Gas hadn't closed my account when I'd left the property as I had asked them to so the account had been open for 3 years in my name racking up bills. British Gas were very understanding and helpful and I just gave them my move out date and they calculated that actually I only owed them about £50, which I then paid. British Gas said they would email Buchanan Clark and Wells to tell them I had paid and there was no debt in my name. British Gas also advised me that if I had called Buchanan Clark and Wells directly, they would not have been able to accept that I had moved out and was therefore not responsible for the whole bill, so it was best for me to call British Gas directly. From this I would suggest that in your situation you call the company who they are saying they are acting on behalf of and if they say you don't owe anything then I would trust them, but maybe ask if they are able to email Buchanan Clark and Wells to tell them that.