Thanks for the reply, I tried your suggestion (see below). This produces a mirror of the external site (at http://mysite.com/mirror/), but the main IP of the server (instead of the VirtualHost IP) is...
I've set up a forward proxy within a VirtualHost (see below) on Apache 2.2.11. I then browse using mydomain.com:80 as the proxy - I've also tried using the IP address of the VirtualHost...