I found I was crying with the sheer loveliness of it. A truly wonderful piece of theatre, magical and charming.
A tale of serendipity/spooooky coincidence now follows:
The reason I bought tickets for this event was because I'd finally discovered the name of the TV show based on this book, which I'd been searching for since the internet was invented. I'd been pleasantly terrified by this programme as a child but could remember nothing about it except that there was a boy ill in bed, a girl who visits him, a magic sketch book which could summon up for real whatever was drawn in it, and some horrible big monoliths outside with eyes on them. Last November I finally tracked down the name of the programme - it was "Escape into Night" and it was based on a novel by Catherine Storr.
Quite by chance I read in my free copy of the Waitrose magazine that the book was being dramatised. I borrowed the book from the library and found it gripping and fascinating, and of course I ordered tickets for the show.