NT Live cinema tickets can cost up to £20 per person, but with a 'tastecard+' trial (£1 for 3 months / £0 for Unidays) you can then buy a cinema ticket code online for up to 40% off.
In my case, I bought a pair of Cineworld tickets for £6.45 each from the 'tastecard+' cinema website. You must purchase two vouchers each time! Then paid 50p per ticket on the Cineworld booking page to reserve seats.
So, £1 'tastecard+' trial + two £6.45 ticket + two 50p booking fees = £14.90 for a pair of tickets (£7.45 each).