They claim that a Notice to Driver was affixed to the car windscreen, was it? In those circumstances, if they want to hold the keeper liable, as per Schedule 4 of the Protection of Freedoms Act 2012, the relevant period is:
"the period of 28 days following the period of 28 days beginning with the day after that on which the notice to driver was given."
To help us help you, please read the following thread and provide as much of the information mentioned in there as you can: READ THIS FIRST - Private Parking Charges Forum guide (https://www.ftla.uk/private-parking-tickets/read-this-first-private-parking-charges-forum-guide/).