I took out an interest free balance transfer on my Tesco CC which ends in September. I pay off the full balance of purchases every month yet for the last two months I've been charged £17 and £28 in interest. It says it's 1.08% on purchases but how can this be when I'm paying the full balance on purchases every month and have 0% on what's left? I just can't work it out. Can anyone enlighten me?