Nationwide Interest

Fingerbobs

Probably what they were saying was that interest starts accumulating as soon as a purchase hits your account. That's the same for all cards. That interest is then waived if you pay the full statement balance by the due date. So technically you are "charged" interest on all purchases, but that interest is later waived if you pay in full. 

cymruchris

In the month prior to them 'charging interest' - did you spend anything on something different to your usual purchases? Did you pay for a service using the card? It might be that you've (not intentionally) used the card to make a 'cash like' transaction, they accrue interest from the moment they hit your account.