they claim that the vehicle was parked on their site for 3 hrs 42 minutes.
Do you dispute this? If so that could of course be the basis of an appeal, if you can produce some evidence that suggests the vehicle was not there for the time claimed.
From a quick look at the notice it would appear to comply with PoFA, enabling them to recover the charge from the keeper.
Other avenues to explore include the signage at the site, if you can demonstrate that it was not suitably prominent etc., do you have any photos?