Here is a link to their parking policy. Note the site manager is key. They say
"If customers anticipate their stay will exceed the stated maximum parking time, we would encourage them to speak to the restaurant manager in advance."
They also add:
" In instances where a customer feels that a Parking Charge Notice has been issued in error, we would ask that they contact our approved contractors to appeal the charge."
We rarely see these cancelled by the contractors as there is no money in it for them, so you should still go back and get the site manager to get involved.
Example Signs [Met Parking]