Unfortunately the 'six year rule' is a voluntary code so the lenders can (and do) make up their own rules to suit themselves, like it beginning from the point of sale.(posted 8462 days ago)The lenders claim that in law they have 12 years to pursue mortgage shortfall debts, which is what your lender will say to you if you query the time elapsed. However, this point has yet to be tested in the Court of Appeal. Abbey National were going to test it last year, against a couple called the Holmans, but mysteriously settled at the last minute. Abbey even paid the Holmans' costs. Read into that what you will.
Like Lee says, read the Repossession section of this web ste thoroughly. It's a minefield of useful information.
I would add that after all this time it is worth thinking about whether or not the lender has even kept the original documentation for the mortgage account. It's going to have a hard time proving this claim if has disposed of original records. The figure it quites is very high, and very specific. How does it account for this figure? Are you satisfied with its evidence?