Totally disgusted, my time is precious thanks. Give me my money back and how about paying me £100 an hour for the inconvenience. Seems fair.
It's just a demo, you'll have the rest of your life to play the game, you're not that important, and if you are that upset then get a refund and boycott valve for the rest of your life because you didn't get to play a game demo 4-6 hours earlier than you were initially told.
In the meantime, if you cry a little more and tell everyone how much you have been hurt and inconvenienced over not getting to play a video game demo at a specific time, someone, somewhere, someday, somehow, might agree with you, somewhat. Everyone else will just point and laugh at how incomprehensibly absurd you are.