Many thanks, much appreciated

Help I'm new to all of this! I have today received a parking charge notice from G24 Ltd:eek: We were visiting the riverside retail park and we checked the time that the car park shut as being 11pm. We were there for a total of 200 minutes and the notice states the time permitted is 180 minutes. To be honest both my friend and I did not notice a time limit for this car park. The fine is £40 if I pay in the next 14 days then goes up to £60 if paid within 28 days. What shall I do? Help