What was the arrangement at the time about how the �1,000.00 cashback was to be dealt with? Was it by way of a lump sum, as is the usual, or some other arrangement such as anumber of deductions from your mortgage payments.
Was it to come to you direct for you to pay to the solicitors or to be paid direct from the lender to the solicitor.
If the former, then, did you get the cash back? If so, then was it paid to the solicitors and, if not why not. If not then I'd follow up with the lender to see what happened.
If the latter, check with the solicitors as to the arrangement they had with the lender. I would assume any cashback offer would have been detailed in the mortgage offer both you and they would have received.
A copy should still be on the conveyancing file (not the deeds) which would have to be kept in storage for at least 6 years. If you used a broker, they should also be able to help.
Make sure they have checked their banking records to confirm it never came in (eg an error in it being credited to the wrong account etc...) and check whether any attempts to chase the lender were made.
I'd ask them how the error was made so you understand, for example, was it an error in calculating the completion statement or different amounts of money being sent or received in error and not picked up on.
I do wonder why it has taken so long to be picked up on. It is likely to be a debit on their office account (ie their money as you can't overdraw a client account as that would be a breach of accounting rules like keeping uncessary funds in credit on office account) which has just been picked up in an audit of unarchived files.
It may be the money is still owed and needs to be paid by someone but, at the least, you deserve a full explaination as to why it is outstanding and from who.