Terminated contract

I had a 1 year contract with Virgin Active which I honored throughout the 12 month cycle. I decided to terminate the contract as I was relocating from the area the gym is situated. I went to the club house a month before the 12th month to terminate the contract and I was told I could only do so when the contract only has less than 21 days before the end date. I then went back a few weeks later within the stipulated time-frame and I filled a form that was supposedly for termination. I was then called by their call center and I explained to the caller that my contract had been terminated which she recorded in her notes. She assured me she would take it up with the club. A month later my account was debited again and I again reversed the transaction at the bank, I was called again and I explained to the caller what I explained to the first caller. Lately I've been getting letters from their lawyers to pay an amount of 1200 which is 3 months premium after the contract had been terminated. When enquiring at the club I was told they do not have my termination documents so Im liable for the amount charged after the contract had been terminated