From my extensive stalking reading of the yr13 thread last year, I think results day goes something like:
From midnight onwards: possible congratulatory email from firm. Not always though, and as Ursula said, some send them to insurance holders too so could be misleading.
From 6am (ish) onwards: track updates, changing conditional offers to unconditional (if you get the grades, not sure what it says if you don't)
Not all unis will have updated though and there are always some glitches, so still not completely reliable.
However a fair few DC will know by this point if they've got in to firm or insurance.
From 9am(ish): schools/6th forms/colleges open so actual results can be collected and we will all finallly know for sure and can stop speculating.
I think it's going to be the longest morning in history and we'll all be either on the gin or champagne by breakfast time!