I definitely want to read the thing before deciding on going or not, since it would be a biggish outing. The film is actually a live feed of the show that would be going on in the theatre that particular day.
I signed up for some sort of youth card for the NT in June, when I was looking at the Phèdre showing, and then it arrived in September, so I know I should be able to get hold of a couple of cheap tickets every time (but then add in considerations of transport to London and time spent out of Cambridge etc...), but it means I'm keeping an eye out, at any rate. If any of you would consider going to shows in the National Theatre (and probably live in London or close by or would be willing/able to leave Cambridge in term time), it's definitely worth signing up to the scheme. It takes ages for the card to arrive, but the first ticket is free, and thereafter £5, I think, with a "mate's rate" ticket at £7.50. I think it's 16-25.