Total deposit £3000. Average balance £1500. Interest at 5% on average balance = £75.
Total is £3075.
The actual interest may vary slightly depending on the timing of deposits and the number of days before deposits clear and earn interest
That figure of £81.25 will be based on 13 payments of £250- e.g. from 1 Dec 2010 to 1 Dec 2011 inclusive.
I get exactly that answer on that basis:
http://www.google.co....B333&q=3250*+0.05%2f2