With some cards you may have to book 2 tickets on the Cineworld website, use the voucher to get one free, and then pay for one ticket - still good at BOGOF.
1) Click on the link
2) Enter Your details
3) Enter 54345 or 54046 for the Mastercard number
2) Enter a random 9 digit number for the booking reference (Needs to be unique)
You should then be emailed a free Adult price discount code to use on the Cineworld Bookings page... http://www.cineworld.co.uk