Mary
I was delighted when I saw you were forming a group. I had previously encouraged customers of vulture funds to do so, but no one took any action.
However, you objective makes no sense. There is no moral reason for Mars to give you a discount. There is no business reason for Mars to give you a discount. There is no legal reason for Mars to give you a discount.
It is just possible that Mars might give you a discount for early payment of the loan. A bit unlikely, but well worth asking for. Don't waste your time or their time or annoy them demanding to settle it for the price they paid for it.
Given that you are not in negative equity, I doubt that they will give you a discount.
Don't tell them that you have gone sale agreed.
Contact them and ask them nicely if they would give you a discount if you pay off the mortgage early. You never know, but they might. I would guess it would be around 5% if they do.
Brendan