It would have never have occurred to me that it was my ISP (Virgin UK) blocking me from downloading cover art! But then again, they have blocked me from using my favourite bittorrent site and I suppose getting cover art for mp3s could be linked to copyright infringement, so it sort of makes sense. Though it's a bit disheartening knowing of the lengths they'll go to, to inconvenience you a bit.
I was downloading cover art through Mp3tag without problem on a different internet connection in a different country; It is only now that it has become a problem. I originally assumed a bug had developed within Mp3tag or something had somehow gone wrong with my computer!
I'm now going to give Album Art Downloader, hopefully it uses a different method to Mp3tag that means the art will not be blocked, otherwise, it's back to manually getting it from an internet browser!