I've identified the 'Latest Poll' block in the centre column as the problem. For some reason in IE6 it seems to be too wide for its container div and this means the last column can't fit in to the remaining space in the overall container 940px container div.
During development of the site I test using Firefox, Chrome, and IE7. I only discovered this problem after launching the site when I ran the home page through netrenderer (http://ipinfo.info/netrenderer/) which lets you see how a site is rendered by older versions of IE.
I've been fiddling with the CSS to try and fix it, but to no avail. If any CSS expert out there can see what's wrong I'd be grateful if you'd leave a comment!
You might wonder, why is this a problem, since surely no-one still uses IE6, which was launched in 2001, but in fact, according to Google Analytics, 6% of visitors do still use this ancient, and problematic browser.
