Thanks guys.
No real trick to it, just sheer bloody mindedness in tweaking and adjusting over and over again (could still do with more). Very ordinary rig - bones and attached skin. The only unusual thing was doing the tail as a separate object using cablecraft and contortionist to distort it with an animated spline. Same technique I used for my earlier snake (posted this link before on CGTalk, but not here):
http://web.me.com/dave.woolard/recent_ads/viper_%28small%29.html
To GeordieJames - urgh!
DaveW