No responses but I have resolved this.
I am using a plugin called WP External Links which can make all the external links on your site open in a new window/tab. They also have javascript code to make all external links open in a popup browser window (which you can set the size for).
I had not idea how to add the javascript to my site so I am using another plugin for that called WP Custom Header Footer – which cannot have been more easy to use.