I got one once. I ignored the 4 letters they sent and never heard from them again. So long as it's not the council or police who ticket you they can't do much.
Even if they did take it to court, which is highly unlikely for £100, they'd have to prove that you being there for more than 2 hours caused them loss of business equal to or greater than the charge, basically, if there was more than one empty space in the car park after the 2 hours were up, they didn't lose business...