So just to come back to this with an update: I managed to score one of the free RPG day books, and ran my first ever session last night. Apparently I did a good job of it, although I got carried away at the start telling people all the details they didn't need to know (extra bits of info, wounds left etc).
I really enjoyed doing it, and it was lots of fun. But I am wondering where to go from here. I have the In Defence of Innocence book and was looking at getting the In the Gutter book, just to help get me rolling, as one of feedback comments last night was it would be more fun and involving if people got to make their own characters.