I went there today and they gave me one of these, basically if you buy one ticket of any type then you will get a voucher to get BOGOF the next time you go.
The voucher you will get will depend upon the type of ticket you buy, so e.g. if you buy a full adult price ticket then you will get BOGOF for 2 adults. If you buy 1 student ticket then you will get BOGOF for 2 students.
Useful if 3 people or more are going because you could just buy the ticket, get the voucher and then use it to buy the other two tickets etc. The vouchers are like numbered gift certificates as well meaning that they don't have any expiry date on them and are valid for all showings of all films including 3D features (with a £2 surcharge).
Not sure how long this will last for, probably until they run out of vouchers I suppose.