little al
Crystal Palace fan
It costs relatively little for a company to refer a claim to the Small Claims Court (£75 for anything under £1500). I doubt they'd bother for a balance owing of £75, but if they ramp the fee up due to non-response / non-payment (after forewarning you in writing) and it amounts to a few hundred quid, then you could well find that someone in the Morrisons finance area will do the necessary, fill out the claim form and apply for Judgement. I've filled out Small Claims court documents in the past in my line of work. It doesn't take very long, piece of piss in fact, not much effort at all, and if the defendant doesn't bother contesting the claim, then the Court will give judgement in the claimants favour and order the fee be paid (and you'll also be liable for costs on top). Of course if you then DO contest the claim, then you'll have to have grounds for contesting it. Not sure whether "I didn't see the sign" is going to stand up really, but then that'd be for the Court to decide.
Its their private land and they are within their rights to make penalty charges, and have them legally enforced if necessary.
He is in a win win situation. Take my advice and not pay, or get a court order and I will pay it.