The address for the Abbey is as follows:(posted 7711 days ago)abbey National Abbey House 201 Grafton Gate East Milton Keynes MK9 1AN
Take Mark's advice and read the site info very carefully before you put anything in writing.
There are various people at the Abbey who might respond to you including Norma Sidebottom and David Huphries who are not as smart as they think they are (personal experience).
Make sure you don't admit liability but at the same time keep your letters courteous indicating that you are trying to be of assistance ot them providing they supply you with the relevant documentation.
Be prepared for protracted correspondence but keep calm and don't let it get on top of you.
When you get your info from the Abbey Sarn, SARN everyone else mentioned in their info.
If your home was undersold,explore the avenues available in order to demonstrate that they did not get the best price. Find out if they used a local estate agency, how many viewings there was etc. Ask the Abbey directly how many viewings and offers there was and then ask the estate agent the same thing......you'd be surprised how there responses may differ. All this might help demonstrate how they are not as squeaky clean as they think they are. If they lie about simple things like this ....what else do they lie about.
Check out the estate agents charges too. The one handling mine charged way over the top I thought. But how do you prove this??? There are ways providing you have a brass neck.
Keep in touch with the site and you'll keep same(ish!).
Take care
M