From the booking form...
A £35 facility fee is payable for each military vehicle entry. Each vehicle entry entitles you to two adult passes
Extra passes above the 2 adults are available @ £30 per adult, Children are free with an adult (0-15 inclusive).
Support vehicles – There will be a fee of £30* for anyone bringing a support vehicle and these will be parked in the designated on-site parking area.
So, I read that as £35 per MV entered - which includes 2 x adult passes, with Children under 16 free... and £30 per support vehicle, with £30 per additional adult not on the MV entry.