I would be scared of this too Skint.(posted 7596 days ago)did your previous full and final settlement letter include 'Without Predjudice' clearly on the letter or state that you dispute their claim etc.
If it didn't then you have already reset the counter i'm afraid for another 12 years (for you not your partner - he would only be effected if you had paid money over) and really you have nothing to loose in YOU (not partner) replying to their form. If you did then I would be very careful about completing the form.
You mention that the property was undersold and that you have considerable evidence - realistically is the undersell money greater than the shortfall claim, did you have an independent valuation carried out before repossession that can substantiate this.
Based upon past experience with this lender I would be suspicious of anything they send to you, it would be their cheek to pay you a peanut compensation and continue pursuit of the shortfall, then if it went to court your undersell argument wouldnt be considered.
Please clarify