Thanks for the reply UB. No it doesn't make sense. The charge is disclosed up front and that's fair enough but to be FAIR to the consumer, me & you, the charges should be displayed on the basis of a credit debit account. The Balance is either a credit or debit to the position you're in at the current time.
E.G. You've had a good 8 months when your first charge becomes applicable, sobeit they debit your account the extra %.
One month later you've had a horror run and have paid in excess of the 20% in charges, then your account is credited BACK the Premium charge if it was no longer applicable to your current Gross Profit/Charges ratio.

It's about equitable fairness, that's all I'm asking for! If you're a successful long term punter, they'll get their cut, but if your success was short lived and you've lost all your Bank plus some, then they should return the charge.