Not fixed at all, just look at the winners.
" just noticed today that somehow they 'know' that Friday's jackpot will be £107million whereas as at 3.30pm today the jackpot is £75million. So, somehow they know that no one will win the jackpot tonight. Or - if someone DOES win £75 million tonight, do they put up the entire £107million on 2nd December from a separate fund? " - That's simple an arithmetic extrapolation of what it would be if it's not won tonight. Probably says that somewhere in the small print.
Occam's razor people.