While looking for something else entirely, I chanced across a drawing of the inside Joy Valve Gear for a Webb 18" 0-6-0 loco, also nicknamed "Cauliflower".
Could not resist the temptation to retro fit my Cauliflower model, so, with the help of Mike Adams tutorial on creating valve gear using 3ds max bones,(UKTS File ID 26781) I got to it.
Three days and considerable hair loss later, I'm proud to display the results of my endeavours.
Mike's tutorial doesn't cover the Joy Valve Gear, but without the hints and tips I got from his work I don't think I'd have cracked this one. So, thanks Mike.