Beware of Euro car parking ticket 'scam' Exactly the same experience. Bought some things from the Esso service station (who own this land) and got hit with £100 fine for 30 mins. No clear signage that car parking charge applies. Have made complaint to Esso and appealled the ticket.
Having read moneysavingexpert website, basically this is a scam - company have rented the land from the services and are sending out speculative invoices. Don't admit liability if you are the vehicle keeper (any contract would be with the driver) and don't pay.
For details and appeal letter template, See: https://forums.moneysavingexpert.com/discussion/4816822/newbies-private-parking-ticket-old-or-new-read-these-faqs-first-thankyou/p1
