Catalogue interest rate charges

Andy20_2

Please can someone advise re the following:-

My wife has, for a while, been purchasing items from a catalogue. I recently wrote out a cheque but on looking at the statement was appalled to see a charge of £50 had been added for interest for one month.
After calculating the Apr this came to nearly 40% !!
Nowhere was the interest rate shown on the invoice. In fact my wife was unaware that interest was being charged as another catalogue she has operated for many years does not charge interest.
Surely it is illegal not to refer to monthly or apr rates on statements?
I would be grateful for any comments e.g. is it legal - who do I complain to? etc
Thanks in advance.

System

fashionworld/simply be [jd williams group] are 39.4% interest, it tells you in the catalogue. i recently paid my acc in full, the most i ordered was £200 but the charges on that was enough to make me want to get rid.