I spent a good hour or two yesterday trying to get this site to validate once again.
When I first set it all up, it was fine as WordPress validates by default. But after several months of continuously messing around with the .php files, I ended up with a botched together mess.
At least I've taught myself a few bits of code... after hitting 7 errors, I realised I had misplaced some end tags. But all is well.
So - click on the small buttons on the sidebar to check for validation. They look like this:
