Welcome to the community forums
Sorry to hear that you have a billing concern. When you have no direct debit set up, if you miss the payment due date, this will show as a late payment fee on your next bill. If you then have not made a payment before the next months bill generates, it will show that you still owing from the previous months bill creating a double charge on the bill + late payment fee of £7.50.
Do you make full payment before your due dates? It could explain why you get bills closer to £40 if you are on a £31 package and a £7.50 charge is added for late payment. You can see your due dates on all your bills and check them against your bank statement to double check.
Let me know if this has helped resolve your query and if not we can support further.