First thing's first this is 100% legitimate, you just need the brain's to search google.
Secondly, this is taken of a forum's I go on called freesteam.org < check it out it's ace.
So this is to play HL2 mod's without HL2, instead you use the demo version of the game because you only need the HL2 engine which the demo provides... Basically when you use this hotfix, when you restart steam you will download the HL2 demo, a small file packed full of fps gory goodness. If you have any error messages when you try to load a mod/game on steam, you're an arse and can't follow step's don't cry to me.
1. Go into your steamapps folder
2. Go to sourcemods folder
3. Locate <game name> folder and open it
4. Find a txt file called gameinfo.txt
5. There will be a line called SteamAppId, this will be under FileSystem
6. Change the appid from 220 (which is HL2) to 219 (which is HL2 Demo)
7. Restart steam
8. Thats it
This is true a fewmod's do, but not enough to make this fix mute.
And I believe you get Source SDK for buying a £4 pound game?
You get it with any source game.
But lot of mod's require hl2 installed.
That's what this fix is for.