I figured I'd shut the site down for a couple of hours to encourage everyone to do their Xmas shopping. The error message was a subtle hint, that's what "500 Error" means ... go shopping.
Whatever rumours you hear, it definitely wasn't that someone deployed a quick update before going on a 1.5 hour car ride to pick someone up and didn't think that the page they tested, to check it was all working after the update, happened to be cached. No siree bob, not that, na hah.
Anyway, you're welcome for the extra chance to, erm, shop.