We've seen a drop in both natural and PPC visitors and conversions in the last week or so. What we are trying to work out is whether this is an industry-wide trend, or if we are performing especially poorly.

What I really need is something that will give me recent Google Trends data (don't we all!). I have been using last year's Google Trends, and competitor's Alexa data as a proxy, but as most of the players in our industry are small and UK based, it's hard to get much from Alexa or most of the other tools (SpyFu, Compete, Quantcast, etc.) - they either don't track the domains or don't have enough data to be meaningful.

Any ideas on how I can get a better grip on search traffic and web traffic trends over time for a whole industry?

You could try the free data of Hitwise's data center at http://www.hitwise.com/datacenter/ or the free data that Nielsen Netratings provides at http://www.nielsen-netratings.com/