Sorry for the slow server / "Server Busy" issues this afternoon. I've only just had a chance to have a proper look at it and it turned out to be some sort of site scraping or denial of service attack. Basically a bot based in Serbia was hammering the site loading hundreds of pages at the same time.
I've banned the originating IP address at the firewall and the server load came straight back down to normal. Hopefully they'll get the hint and 'eff off now!