Very simple... You were not parked overnight; you parked and boarded the first train. If they refute this, ask to see CCTV evidence. Not sure how you can prove overnight parking at 0820 in the morning?

It's issued under the Merseyrail Byelaws (Byelaw 14 I believe) and is an administrative penalty. You can choose to ignore it if you like, however it will result in keeper details being obtained and then turn into a court date in front of a magistrate - completely different to private land "invoices" and council "fines", and will end up in you paying the full ticket amount, plus costs. I suggest you pay it, and learn from your mistake, unless you feel that you have a strong case to appeal it in front of a magistrate?