Just got an email through from PureGym as I used the 3-day free pass that was on offer here at the start of January.
Just enter code 'NOJFJAN' in the promo code of the sign-up section.
I received this code as I had signed up for a gym in Glasgow City Centre, so it MAY be location-specific, although I doubt it.
Grab a bargain for the new year! Be quick though as it says the code expires at Midnight on 19th!