How can I work out where web site visitors come from by their IP
address? I.e. I need an automated way of working out which country a
visitor comes from so I can redirect them to a particular country
site. E.g. visitor goes to "x.com" but it automatically redirected to
"x.co.uk".

I'm looking for code to implement this or a bit of software which will
do it. I'm aware that "I could" do a whois lookup for each IP and see
if a location it assigned to it.

You should probably check out http://www.geobytes.com/