My Gym hasnt charged me once, who's at fault?

Okay so I joined a no contract gym membership 7 months ago and apart from the initial joining fee they haven't taken any money out of my account even though they are supposed to every second week.
Im not sure for the reason, I havent used the gym more than a handful of times.
I'm uncertain to if when I joined they had a 6 month free thing or something because my friend who joined a week after is in the same situation.
He also wrote me down as a referral so I would get 1 extra month free

what should I do ?
Whos at fault? and can I possibly be back charged for them not taking the money out?

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