I agree with Lee, but I would just add as a discussion point that there are some signs that lenders' valuations on properties were hopelessly optimistic in the past (even after the boom-bust of c1990) and did not take into account that all important factor, Location Location Location. Borrowers are usually in no position to sue for negligent valuations, because (a) they are skint and (b) the valuers are usually not direct employees of the lenders, and it all happened over six years ago so (ironically) the Limitation period has expired.(posted 8503 days ago)Are you at all in a position to take on another mortgage where you are moving to? I recall some lenders offer 'negative equity mortages', where the new mortgage absorbs the previous loss, but if you are moving, say, from the north to the south-east then you may not be able to afford a mortgage at all these days.
Whatever you do, document everything and keep every single piece of paper in a safe place. Show that you have made a genuine effort to resolve this with your lender.