if the make of car on the ticket is wrong, then it is void.
if you go to the parking shop to enquire about it, do not give them the proper details, as they could try and still make you make.
as regards to the times, parking attendants are legally obliged to give 6 minutes grace for the owner to return. check your ticket!
and if the appeal takes longer than the 14 day discount period(which will any way, because you cant appeal untill after the 30 days), and you lose, then chances are, you will have to pay the full amount.