This has just started happening to me after TF2 has almost finished downloading the required files for the HH servers.

Any ideas on this one?

Verify integrity of game cache
https://support.steampowered.com/kb_art ... 5361#basic

Done, and it's still giving me the same error.

The game was running fine last night too, and I haven't touched anything since then, so this is an odd one.

TF2 isn't meant to have a surfaceproperties.txt in the scripts folder. Delete it. (or back it up and delete it.)

I think I may have to reinstall the game. Bummer.

Nope, you don't have to do that. Try removing that file first.

I'm getting similar errors but for different files when attempting to log into other servers too though. Something aint right...

Sometimes things like that (for different files) have been happening to me after having previously joined a pure server or something with some mods*. Just rebooting TF2 has fixed it every time. It's been quite recent as well.

* don't ask me for details, I have no idea.