In the absence of advertiser chosen regional targeting, AdSense will look at your content for inspiration, then at your domain extension and probably hosting location too, what you are doing is confusing Google with dynamic content and domain extension. If you are targeting the US, get out of the .uk domain also while you are on it host it in the US and keep it static whenever possible.

I'm pretty sure G displays ads based on the user's location. You would not need to indicate this. To see what adverts are being served to the USA, you might wish to try the ad preview tool:

[google.com...]

Thanks Hobbs,

But if I open up a .com site with almost exactly the same content, just a few differences caused by the asp code, then Google will think I have two sites with almost identical content? I don't want to abandon the .co.uk site because it's a good earner. I just want to extend it gradually to include the USA market.

As an example, if the viewer has entered his town as Blackburn in the UK, one line of the page might read 'the weather in Blackburn is not cold enough to play ice hockey outside'. But if the viewer is from Alaska in the USA, the same line of the page would read 'the weather in Alasks is cold enough to play ice hockey outside'. The rest of the page is fine for both UK and the USA.