I used to work for comet about 4 years ago as an Admin Manager.
I would have thought your purchase would have been on a 6 months interest free Option as they called it. Meaning you would make 6 small payments and then have the option to pay off the balance by the 6th month which would then be 0%... Otherwise they back date the interest to day 1 of your purchase and your on the 60 or 48 months payment.
So I would say from your comment that you are just paying the small direct debit to them each month?
Comets finance company always sent out a MyWay, Edge Card or Timeplan card when someone purchased something on 0% but they probably failed to explain that in the shop, and that its about 26.9% interest on any balance you put on the card and dont pay off straight away.
contact GE Money directly who operate these accounts, and they will be only to pleased too assist you with your enquiry. For your information their details are as follows:
GE Money
Trent House
Torre Road
Leeds LS99 2BD Your Way / Timecard 0870 125 2506 Edge card 0870 126 2608
make sure you have your agreement from your 6months purchase and your Comet Card to hand when you phone as this will speed up them finding out about your account and whats happend.
Hope everything goes ok for you
Mike