From the looks of the PCN you've received, the car park is enforced by ANPR cameras rather than wardens on foot, is that your understanding too?
If so, the first argument that springs to my mind is that there is no commercial justification for requiring payment within an arbitrary deadline upon arrival, making a £100 charge for paying 'late' (but still before leaving) an unenforceable penalty.
The ParkingEye vs Beavis case in the Supreme Court held that parking charges that would ordinarily be penalties are acceptable in some circumstances, where there is a commercial justification. For example, in a free shop car park with a time limit, there is a commercial justification for a charge for overstaying, in order to deter misuse, which would block spaces that genuine customers could otherwise use.
Here, I can't see what legitimate purpose there is in requiring payment to be made within a certain time after entering - all such a term would seem to achieve is increasing the chances of issuing PCNs. If the car park was patrolled by a warden on foot, I could understand needing payment on arrival, as you'd need to display a ticket so that the warden could check who has paid. With an ANPR car park however, they could simply check payments received vs time spent on site, and allow drivers to pay at any time before they leave the car park.