www.payasugym.com
Came across this the other day and thought I'd share.
Handy for those that don't want to get tied down by a gym contract or can't always get to a gym regularly. Not sure about other areas but they seem to have quite a few gyms in my area that you can pay for one off days
Also on Tesco clubcard too
http://www.tesco.com/clubcard/deals/product.aspx?R=2860
Exchange £5 clubcard vouchers for £15