I hope this links works. They sent me an e-mail about this.
Just print the voucher and take along to nearest Gym 8am-8pm. No appointment needed. Also if you join this weekend there is no joining fee.
I used to belong to JJB Gym and they are really nice-the swimming pool, hot tub and sauna make it worth the trip! (I only cancelled membership because I moved and there isn;t one)