That's right - the account number starts with "40". The margin is not mentioned anywhere in the loan offer.based on the pic you have provided and the account being drawn in 2008, and the mortgage being sub account no 1, the mortgage number should start with 40******.
should they not have had the benefit of the dropping rate as it was a variable rate offer, this would have been the norm and the loan should have drawn down at the prevailing variable rate at date of drawdown. If rate had risen they would have been charged the new higher rate.
[broken link removed] Here is a Special Conditions page.check the special conditions page of the loan offer to see if there is a margin shown there.