You may like to send Eversheds a copy (or just refer them to it) of an article in the Sunday Times 'Money' section, dated 08/03/98, in which a Nationwide borrower won a very substantial refund.(posted 8612 days ago)You should put them to 'strict proof' of the debt, i.e. ask to them to provide detailed (very detailed) information as to the calculations. You should also follow the info given to you on this site. Your offer correspondence should be without prejudice. Don't be bullied - be strong - it is your rights that you are seeking to enforce.
Good luck. Ken