I haven't helped things by wrongly thinking you had parked during the "Overnight Parking" period, so I do apologise for that.
Here's the sign with some mark-up:

Essentially, different rules apply at different times. First:
2 Hours Max stayDuring the times indicated (Mon-Sat: 7am-10pm, Sun/Bank Hol: 11am-5pm), parking is free, but drivers may only park for 2 hours or less.
Second:
Tariffs apply outside of these timesIf you park outside those times, the 2 hour max stay does not apply,
but, there is a tariff to park, which is £15 for up to 9 hours.
Your car was parked from 18:00 to 21:01, on Saturday 16th March. This was during the times indicated on the signage, when the maximum stay was 2 hours. You breached this term by staying for longer than 2 hours, contrary to the advertised terms. Therefore, the following applies:
Failure to comply with the terms & conditions will result in a Parking Charge of: £100You can seek to argue that the signage doesn't make this clear - the hurdle you will need to overcome is that the bit I have highlighted in red does make fairly clear that between 7am-10pm Monday-Saturday, you may only stay for 2 hours. You could try to argue that "Tariffs apply outside of these times" could be interpreted to mean that tariffs apply if you park longer than 2 hours - whether a judge would agree with that I don't know.