Richard,
The devil is in the detail, which started first, the legal or the negotiations. I should point out that the bank are entitled to start legal proceedings against you once you have defaulted on the terms and conditions of your loan agreement in relation to a buy to let property. However, having said that, as soon as you signed the split mortgage deal with the bank, the bank have effectively resiled their litigation. They are entitled to recover monies expended by them regarding legal fees up to the date you signed the split mortgage deal, but no more. My advise would be to look for a comprehensive account of the legal fees outlayed by the bank, paying particular attention to the dating of the various fees. Then enter negotiations with them to come to a mutual acceptable agreement. For the Bank to attempt to charge you interest on these legal fees they must prove to you that their legal representatives are charging such rates. If they cannot do this, you do not have to pay this interest, unless of course, there is such a clause contained in the mortgage agreement.