I did this with a PC from PC World a good few years ago, 9 or 10 months interest free it was (can't remember exactly) but I logged the exact date I set the credit agreement up from and even had it set as a reminder on my mobile to make the full payment at the correct time!

This was because a friend of mine who worked there at the time said they had a habit of not reminding people when the interest period was ending!

Strangely enough I got the letter reminding me, so needn't have put the reminders in place, but I heard of alot of people who didn't and got stung like you have.

I don't think you have much of a leg to stand on really, the way they see it, it was your responsibility to remember when the interest free period ended!

Let me guess - it was HFC bank (I think HFC stands for Horrible Fucking Cooooonts )

Andy