Really need your advise on this one...is it possible to get tickets for the play at the box office for the very same night??? Or do I have to book them in advance online?

I am going to London next week and I really wanna see Josh!!! However, the girls I am travelling with are not that big fans of him and they wanna decide at short notice when to go (or whether to go at all ) See my problem?! Can I get 3 tickets (or probably just one for me) at the box office or is the play likely to be sold out?

I count on your experiences and appreciate any help...

Cheers Ulli