I purchased something on a buy now pay later 0% interest for 1 year agreement 11 months ago. The balance was £3,296.25.
I've just logged into the finance portal to settle the balance before the zero percent period ends and the balance to settle is £3,325.25 
I'm about to pay it so I don't forget next month but I want to email the company asking them to reimburse the £29. Can anyone tell me what the equivalent interest rate is having paid £29 interest over 11 months on the starting balance so my letter sounds even crossed!?
Thanks!