Team Fortress 2 LAN server not visible in server browser

I've got a LAN event on this weekend and we've run up some virtual servers for running all our dedicated game servers.
When I run the TF2 server it doesn't appear in the LAN tab of the server browser.
I have sv_lan set to 1, and I can join to it using the console.
Other servers such as DoD Source are appearing in the browser.
The Windows firewall is off.