I think you're being hard done by.
When I paid mine off a couple of years ago, the building society suggested a date about two weeks after the date I called, so they could work out an exact figure. It meant I had time to arrange a transfer of money to my current account, and send a cheque off to them by normal post - not the expensive CHAPS method. Also, I think because of the length of time I had my mortgage, they did not ask for any further admin charges.
Did you take the name of the person who gave you the original figure? I don't know how long you had the mortgage, but asking for another �200 after giving you a settlement figure seems very harsh to me. They should be able to waive this surely.
I think you need to stand firm on this one, you cannot go to the ombudsman on this until you've exhausted all other avenues. Whatever you do, don't lose your temper at any stage.
Does the BS have a branch locally, if so, go in and have a word with the manager or a senior member of staff;after all they don't want their 'good name' blackened by you around the town - do they?
Good luck - keep us all posted whatever happens.