Interest on Interest Charge for Arrears on Account

I would like to ask a very simple question maybe a stupid one by that. I purchased a TV from Incredible Connection, HI Finance, the purchase amount was R10 000.00 and over that period interest was calculated. My monthly payback is R891.00 over 30 months to a total of R26 730.00. I got into financial difficulty and called them to make arrangements to just take a three month breather to catch up. Over and above the monthly installment there is an additional R250.00 levied for arrears. May they charge interest on interest. It calculates to 28% interest per month. Is it legal.