Surely regardless of the fact he was speeding if the police get the prosecution wrong, i.e. 40mph & not 50 or 60 then its a wrongfull prosecution & as you cannot be prosecuted for the same offence twice. I would personally keep quiet untill court & then make the the police case look the foolish prosecution it is.
The mpv issue could muddy the water even more, leaving one little tax collecting piglet looking even more foolish.