Gym charging problem

WickedWoman

I joined Fit4Less. They have a 30 day agreement of £17.99 - no contract. 30 days notice to cancel.

My Direct Debit went out on 5th each month, paying in advance for the next 30 days.
I decided to cancel on 8th of November giving my 30 days notice in writing. They confirmed that my membership would end on 8th December. My Direct Debit was paid up to 5th December so I cancelled it and paid the 3 days for 5th-8th December with a cheque and made it clear that this was paid in full and final settlement on the account. They told me on the phone that I had to pay the full £17.99 on 5th Dec, despite my membership ending on 8th Dec. They insisted that I had to pay up to 5th January.

Since then, they have started to text me to tell me that my account is outstanding and I must pay or they will pass the debt to the debt collectors.

So although their T&Cs say you are required to give 30 days notice to cancel, they will actually charge you for anything up to 59 days.

I don't think they have the right to do that. I put this all in writing when I cancelled and they have completely ignored my letter, but have banked my cheque.

Can they send this to the debt collectors and if so, can the debt collectors take action against me?