Robert_Charlton - 10:56 pm on Sep 12, 2012 (gmt 0)
JackR - Penguin was about overt spam. Panda, which apparently is what the timing suggests that you were hit by, was about the quality of the site content and presentation.
Low hanging fruit, too many ads, content too much like that of other sites, was hit in the early rollouts. Too many similar pages within a site, eg, built just for targeting different keyword variations, was a common problem. Data sorting problems, too much faceted navigation, also could cause Panda drops.
Panda isn't a penalty... it's kind of an algorithmic filter, meaning that Google has simply raised the bar, and, for whatever reason, your site isn't getting over it.
I'm not sure why you might have dropped when you did... nor even for sure whether it was Panda, but the timing does sound like Panda. I suggest you go back to the very early Panda update discussions and do a lot of reading. Also use site search to look for Panda recovery topics.
You might also want to check out your site in terms of the quality factors and check your backlinks, to see if you spot any signs that would cause a drop. A great many sites simply fail in terms of content.