YG - most people do not go back and re-order photos from old sittings so the new owners would not have been expecting to make more money from the shots they have of your sitting. As such I think anything they get for them would be a bonus!
I would start by offering £250 but would be prepared to pay a lot more to get them all on CD. They are useless to the photography company - but priceless to you.
I think once you explain the situation to the new manager (best done face to face with a few tears if you can muster them up to order) then you should be able to do a deal.
Good luck