home shopping - unfair late/non payment fee

I currently have an account with a home shopping catalogue which requires minimum payment every 4 weeks. Recently, due to my statement date and when my salary gets paid, I knew that I would not have the funds for the following months statement. So in order to ensure I didn't fall into arrears I made two payments for the minimum amount. Then to my dismay they applied a charge for non payment and requested a double payment the next month for 'arrears', despite me having paid more than I should have at this point of the 52 week term. After speaking to customer services they state the consumer credit act prevents me from paying ahead of schedule. Is this true and are the charges legally justifiable?
Any help or suggestions about this blatantly unfair practice would be greatly appreciated.
Regards,
Alan