404 displaying at top of website

Hello,

I'm a new user so not sure whether this is the kind of support you offer, but just in case... !

My client's site has "404" appearing at the top of it in a grey bar - http://ukrainewatch.cftni.org

I've attached an image of it opened up in Firebug and you'll see there's a Twitter related iframe just above it. I've posted this on twittercommunity.com but not had a response

There is nothing in the header file. I've disabled all plugins and it is still there. I've also re-uploaded all the theme files from the last backup and it is still there.

Any thoughts would be HUGELY appreciated!!!

Tom