Working Out Redemption Figure

CPS

I got my outstanding balance from Ulster Bank 2 weeks ago and it was £102k which is what I have been doing my calculations on. I now have my formal redemption figure from Solicitor which is £102,800. I phoned the Bank to find out where the £800 had come from and they advised it was to do with when the interest is added on and the fact I pay my mortgage in arrears (even thoguh I paid a full month's mortgage after only being with them for 2 weeks when I joined last year so I dont count that as being fully a month in arrears).
Any simple explanations as to how this works appreciated

Thrugelmir

The interest element of your mortgage is calculated and charged in arrears. The interest will added to your mortgage account once a month.

Between the time you phoned your lender and the solicitor obtaining the settlement figure further interest has been added. In addition there will be charges for redeeming your existing mortgage within the £800.