UK Law concerning Buy to Let Mortgages

I have a situation with the UK mortgage lender where I have a mortgage which needs to be redeemed in my soul name by the 31st of March 2026. When I asked the lender for a redemption figure they picked up on a disputed joint mortgage with a third-party from 6 1/2 years ago which had been repossessed because it had come to the end of its term what they are saying is that they have the right in law to now link both mortgages so when I repay the debt owed on the mortgage in my soul name they will not release the charge until the debt is repaid on the mortgage in joint names. I have questioned this two or three times and they refused to budge I would like advice on whether this is something that would stand up in court given that the two mortgages are in completely different entities, a little information regarding the joint Mortgage since my complaint about the way the repossession was handled 6 1/2 years ago I have not heard from them at all not one letter a statement or any communication whatsoever. It is my belief as this has been written off and that they are now trying to collect this using the security of the Mortgage in my soul name I have been trying to get this matter resolved them for over nine months they are In London! who no longer lends New borrowing they are just executing the old MORTGAGE EXPRESS book. It would be good if I would be able to speak to the successful person to discuss this matter as well as have a written answer as there are some complicated issues.