Held liable for staff not doing their work

I phoned to cancel my gym contract - which I was told was done, but meanwhile the person on the other end (at the call centre) didnt do their job, and did not cancel - the contract had already expired and I was on a "month to month" basis with them. now months later I received a letter of demand to pay account thats in arrears - suddenly the call centre cannot trace my call back to when I cancelled and are holding me liable for outstanding amounts not payed - how I am being held liable for other people not doing their work?