My personal view is that you should only co-operate with a property sale if there is written agreement on what happens to the shortfall, and ideally that this is written off. In other words, co-operate in exchange for a full write-off of the shortfall. As you have already tried this, and were left with no option but to petition for your own bankruptcy, the "major law firm" and their clients (a bank or possibly the ISI) should be left to deal with the matter themselves. They will probably have to get a court order to allow for the sale.
Sign nothing (really important, there have been instances in England and Wales where former bankrupts unwittingly took responsibility for shortfalls).
Say nothing other than, "I'm bankrupt (even though you no longer are) - all queries to the Insolvency Service".