Simsi - 2:34 pm on Nov 10, 2012 (gmt 0)
I didn't mention in my original post that after Panda 2.1 I developed 4 similar sites but all with unique content. Different hosting accounts. Two had Adsense and two didn't. The two with adsense originally ranked very well but have disappeared with Panda 20 / EMD. The two without adsense have kept their ranking.
Not wishing to sound insulting - especially as I don't use Adsense myself - but is it maybe possible that Google looks at the quality/ratio of visitors on a site that do/don't click adsense ads, plus how they respond to the ads (buying signals?) and factor that in as a "quality" rating? Just musing.