k, as we've all found out, I'm not managing to host lobbies anymore.

Google tells me I'm not alone in this regard, many people having the same issue.
A lot of people having problems similar, especially those on same LAN as server.
Now, I'm not on the same LAN exactly as the LS server, but I'm permanently connect via hardware VPN back to our office (for IPT, internal access, etc) so it could be VLANs and routing getting themselves all twisted.

So, research into alternatives has found an alt solution, that you all can use too.

Create a blank file, based on your L4D2
D:\Games\Steam\SteamApps\common\left 4 dead 2\left4dead2\cfg\autoexec.cfg

and enter just:

mm_dedicated_force_servers 89.104.234.121:20002


Might be pointless, but it should deffo work.

Had a look into this - it could be worthwhile disabling the "Find local LAN games automatically" (or similar). I've seen reports that that might help.

Bumping

FWIW the disabling local LAN games did exactly exactly nothing for me.