With a bit of modification a Halfords ball joint splitter made an easy job of getting the handle off. I never did manage to completely manoeuvre the mechanism into a place where I could work on it easily but I got it just far enough to do the job (with some difficulty).
I found that the black plastic clip (shown in Oddyseus's photos) had broken. On checking with Lewmar I found that the part is available as part of a fitting kit which includes the two clips, two cable nipples, two split pins, various fixings, a replacement rubber cap for the centre of the lever, a replacement mushroom button for the gear disengagement thingy and a tube of superglue for fitting the rubber cap. Delivered in just over 24 hours (that may have been a bit of luck as the Lewmar rep was in the chandlery when I phoned) and a total cost of £20.
Everything is now back together and better than ever as I discovered the friction brake on the lever and the fact that mine was wound in too tight. It now glides between forwatd and reverse easily and smoothly. I now have a spare clip and various bits in my bosun's box in case it ever happens again. Thanks to Oddyseus and Landes for the help.