Thanks. Not sure that I've proceeded with caution ...
I've taken it back to bare roots - hopefully I understood that correctly! Many of them were attached to the black plastic pot, so I have cut the pot and left them attached.

---------- Post added at 09:20 AM ---------- Previous post was at 09:17 AM ----------
The crown is not looking good. I cut it back but I'm not sure that I got to any kind of live part. The greenest roots are just back from the tip, so I'm thinking it would be best not to cut back any further.

---------- Post added at 09:27 AM ---------- Previous post was at 09:20 AM ----------
So if I understand correctly, I could put some damp sphagnum moss in, put the lid on (perhaps left open a crack?), keep it somewhere bright (but not too hot), wet it regularly as per estación seca's advice, and see what happens?