It will have to be dealt with as a remortgage and transfer. Depending on which Solicitor you use, they may try and bill you for three transactions. As well as dealing with drawing down the new mortgage and redeeming the existing one, the solicitors will also have to draft two sets of transfer documents, one to remove your father's interest in the property and one to add your new partners details. When I was acting as a conveyancer, I would have looked to charge approx £450.00 +VAT for the re-mortgage and £150.00 + VAT for each transfer, so £800 sounds reasonable if VAT is included.