My gym has a policy that if you book a class/activity in advance and don't turn up you get charged a £4 no-show fee.
This makes sense for people on contracts - clearly they don't want people booking up loads of classes and then not coming, and depriving other people of the spaces.
However, I don't have a gym membership, I pay on a PAYG basis. I recently paid online to attend a class and then attended the class. It appears that my attendance wasn't registered because I've been told I have to pay the £4 no-show fee. However, I'm not sure of the validity of this even if I hadn't attended? I paid for a specific service (attending the class) and even if I hadn't turned up the gym still got my money for this. They surely then can't charge me more money for not using the service I'd paid for? If anything, this is great for the gym -if lots of people pay for classes on a PAYG basis and don't turn up, they are raking in money for nothing.
(I should also say, in case it matters, that the class was only about 1/4 full, so in no way did this affect anyone else).