Thanks Dimitris and Marria. Dimitris I read the documentation but as a non-programmer it made my head spin - I can't work out which things I've done already and which I have to do now.
Marria, re WP
Subdomains, I'm not sure it still works for WP
(8 out of 10 reviews there say it's broken and it hasn't been updated since 2009) plus I don't want to make my categories or authors into subdomains.
I want the same site that I have already, moved from mydomain.com to either uk.mydomain.com or mydomain.com/uk, and then a load of other sites created that are exactly the same, but without the ads, so that I can get new ads on them that apply to that country.
It seems to me that there are 2 main steps here. First - transfer the existing site to one that is either uk.mydomain.com or mydomain.com/uk, and Second - create a load of similar sites but for specific countries. Does anyone know how to do either of those things? Or am I wrong and I should do something else entirely?