Forgotten mobile contract

My husband took out a mobile phone contract several years ago, but when his contract was up he bought a new contract with the intention to cancel the old one. Due to his anxiety and Aspergers he put off cancelling this and forgot about it and has been paying for this until recently.

When I went through his bank statement several years later, we could not figure out what it was and T mobile would not give us any information as we did not know the phone number. Subsequently we had to cancel this direct debit through our bank to cut it off at the source as T mobile had been so unhelpful.

Several months after cancelling this direct debit, he has now received a letter from some company or solicitor, stating T mobile sold his contract to them due to non payment of £68.36 which we now owe them!! We never received any letters asking for money, and as we had paid thousands on a contract he wasn't using, are quite shocked and confused as to where this sudden "non payment" has come from!

Is there any point in us trying to claim any of the money back? How on earth do we go about this?

Any help gratefully appreciated.

Please don't leave unhelpful comments like 'how can yo forget about a DD'! My husband has Aspergers and put off checking his statements for years because it caused him so much anxiety.