Flexi trouble

In April a cancelled my membership with them and I then received a cancellation letter and them offering me a cheaper plan which I was still not interested, now they denie that the letter saying that my membership number does not appear on the letter and I was not cancelled and I still owe them I'm listed on ITC and they refuse to rectify their mistake so I'm taking this further to ombidsman, lawyers and consumer rights guys pls be aware