can they still keep adding fees?

I'm in an argument with some direct debit company - I signed up for a 12-month membership to a gym last year, and paid 1 month in cash up front, and subsequently another 11 direct debit payments. I now go to a different gym so have cancelled the direct debit, but they're claiming I owe another month because the cash payment was to allow me to use the gym for a month before my 12 months started. I'm contending that this is nonsense.

Anyway, they're basically saying that I owe them about an extra tenner a week at the moment while this rumbles on. Is this allowed?