1) The Q&A will be chaired by the Arts Editor of The Daily Telegraph who will begin with a series of her own questions and then invite audience members to ask their own questions directly to the cast-members.
2) It is not permitted to record the event in any way. [I asked about filming and photographs - this means both are not allowed, I guess]
3) The session usually lasts around 1 hour.
4) The cast are welcome to stay and chat with audience members but, if they have an evening performance to prepare for, they may not stay for long.
[edit: I just wrote back asking whether the session would be recorded and they replied (lightning service!) that "there are no plans for the Q&A to be professionally recorded"]
By the way, please leave a note in the comments or forum if you're going; perhaps there can be a mini meetup?