I hope this helps 24 pin ATX has been with us a while and are mainly used in workstation systems
I think click here do a convertor but I have to say I am suprised the case didnt come with a convertor
It might be easier to part with £3.00 for a convertor than to get a refund out of Stak trading
I have never dealt with Stak trading but if I were you I would look to source you bits from elsewhere.
Hope this helps