Next Card interest payments

CG19a

Hi, hope someone can advise. I've had a next card for about 6 years and never really had a problem with it. I checked my statement today and noticed that I've been overcharged on my interest by around about £4. I checked last months statement and have been overcharged by £2. I called up today and they are going to look into it, but can anyone advise what is the best thing to do if they start to get sniffy about it?

On the back of the statement it states -

Interest Charge
If full payment of the current balance, as shown overleaf, has not been received by the due date shown, an interest charge calculated every month at a maximum 2.014% (APR 26.49% variable) will be made on the opening balance on your next statement.

Just got this from their website, anyone know which interest rate I would calculate out from? Is it the 2.014% which I've been using currently or should I use the relevant one below according to which month my statement is in?

The APR rate is calculated at 2.014 % variable (APR 26.49%)

The current service charge rates are shown as follows:

Daily charge (based on 365 days) 0.061%
Monthly charge-31 day month 1.907%
Monthly charge-30 day month 1.845%
Monthly charge-28 day month 1.783%
Annual charge-APR 26.49%