Over night shifts should start well before midnight personally. I think Glasto has it right there doing a 10pm-6am slot.
Midnight-8am isn't really an "overnight" slot imo.
The mini buses are nice enough but a little bit useless as, unless it's changed, they left the oxfield 45 minutes before your shift started. So not really a time saver but a leg saver of you don't want the walk.
But yeah it's usually a good deal.