With most people probably trying to shed some Christmas excess at the moment, why not treat yourself to a free day at Esporta health & fitness clubs.
I am a member at my local one and it's good to have a dip in the pool/spa etc even if you are not a treadmill freak.
For a list of clubs around the country, click on the link and choose from the dropdown on the left.
You need to complete the form with as little/as many lies as you like to have the form with unique reference e-mailed to you. You then need to print this off and take it to your local centre.
NOTE: The form asks you for a phone number. When you submit it says that "an agent may contact you to arrange a time". Put in a bogus number if you do not want this call...