I dunno. "At just 79p per £100 of your Card balance each month, our Plan provides great value for money."

I pay my card off every month, but today there is a £3k+ balance, as I am between statement and payment.

I reckon they hit you with the charge based on your month end balance, regardless of whether you pay it off in full. If you have no transactions in the month and the end month balance is zero then no charge.

Maybe wrong.