Well, several days on, and the suggestion above appears to be doing the trick, so thanks for that. :cheers:
It's interesting to note that another post about what would appear to be exactly the same problem has now appeared on this very forum...
The weird thing is, it was working fine until yesterday afternoon. Done a complete reinstall, deleted the entire Valve / Steam directory from my hard drive for a fresh start, and I still get this problem. And I've downloaded all the latest HL2 updates as far as I'm aware. It wanted to download...