How to change the default site hosted by the ip address of a linux plesk server?

I have a Plesk Ubunutu VPS that had until recently one domain setup. If I accessed the IP address of the server I see the site setup on the domain. The problem is that I've now added a second domain and website and want that to appear when the IP is also accessed in a browser. To change this I've already done the obvious thing by going into the "IP Settings" area of the Plesk configuration website and changing the default to the new domain I've created. This doesn't help unfortunately and the site hosted when I access the ip number is constantly the first site created.