When you say you’ve checked in Google, how exactly have you checked – with Google.com, with Google.co.uk, with google.co.uk with the ‘Uk sites only’ option selected?
If I get an unusual hit, often it will have shown up in a search in a foreign language Google. I get quite a lot of traffic from SE Asian versions of Google and bits and bobs from eg Sweden. When I type in the relevant search term to that particular engine, I often find that I rank differently there to the .co.uk or .com Google results.
I’ve heard that it shouldn’t make a difference, but in some cases it clearly does.
Try analyzing your log files if you need to pinpoint an exact referring source.
Hope that helps a little!