Well - that's it. I give up. I've been well and truely goosed by a panda (we've been dropping since Nov 6th, then hit again 21st and today has been just horrid).
I've tried pretty much everything I can think of including a full redesign/content purge and merge.
The only thing I'm clinging on to now is that during our redesign (launched early Sept) I noticed that we had an enormous amount of "pages not selected" in GWC - likely caused by dodgy url rewriting script - which is now fixed - although all these urls will now give 404 errors...
If anyone can think what I'm doing wrong then I'd love to hear from you.