This is the most affordable way to attend premium classes regularly, costing only £6.25/class! The monthly fee is £25, and it's auto-billing. This means if you buy it on the 4th of January, your 4 class credits will be valid until 3rd of Feb. On the 4th of February you're billed again and automatically get a new set of 4 credits.
What if you want to take more classes than 4 in a month? Or if a calendar month has 5 weeks in it rather than 4? Since each month doesn't have exactly 28 days (=4 weeks), you may notice some months that you'd like to book a class but you have no credits left and your renew date is in a few days time. Not to worry! You can just top up and buy an £8 individual class credit and book on. Doing the billing monthly rather than exactly every 4 weeks means you've got a few extra days flexibility when it comes to make ups (if you have missed a week and have an unused credit).