My son fell asleep, and overstayed the 2 hrs apparently, at a services early morning during a long drive. He had a demand for £150. He refused to pay citing he wanted photographic evidence on arrival and exit times. Which they have failed to do. The parking company kept writing about the court case they had won, re a motorist who also overstayed, and had disputed their claims.
You can also argue the sum claimed is disproportionate To the claimed offence.
In my sons case he cited that there are signs before services “ Tiredness kills take a break”