Thanks Netmeg - appreciate the explanation. Penguin would explain it, though from what I've read my sites have no cloaking or keyword stuffing anywhere. It's all written in natural language. It hasn't impacted my main website at all, which again I find odd, as there's really no difference in the way I constructed any of my websites. My backlinks are all good, and I've no paid traffic, or links from spammy link farms. I don't get it.