I'm not a mathematician either but I would guess at..
Say you spend £1000 on bets and your returns for the month (and being an unsuccessful punter your winning bets are £200).. Therefore £800 is the net turnover..25% of this is £200 and if you apply 8% to that the rebate would be
£16 rebate..
Erm..probably totally wrong..

.. Anyone else want to have a stab?