What I said about the MIG's was from my understanding that the parties are the MIG Insurer and the BS and as such, third parties do not have the right to dispute the arrangement which would include sight of the MIG document itself.(posted 7249 days ago)Underselling is very difficult to prove - all the BS has to do is prove/show that they sold at the best they could get at the time - so an offer shall we acadmically say of £30,000 from a man with cash (i.e. no mortgage to obtain) would get preference over the man who offers £40,000 but needs to fund the purchase with a mortgage. You are potentially on a sticky wicket here.
As to proof, yes, you are entitled to put the lender to strict proof of the debt - I think you will find that they will be able to show the above regarding sale and all the ancillary costs plus the ongoing mortgage premiums from repossession to sale and that it will all add up correctly.
You perhaps ought to be thinking along the lines of making a settlement offer - there is plenty of guidance in this forum from those who have done this - successfully.
Good luck with it.