I understand that when an outfit arrives after hours it is standard practice to retain the membership card until the following morning when fees can be paid and card retrieved.

I can't see anything in the club rules about that ET, it does state:

"If you're going to arrive after 8pm on the day you've booked in, please arrange a later arrival time with the site staff in advance, otherwise your booking may be cancelled. If you intend to use a late night arrival area, please let the site staff know in advance".

Which it seems the OP did. 

I think you overpaid, BD, as if you spent the night in the LNA area, there shouldn’t have been any pitch fee to pay with only the per capita cost due.

Hi all as ferry meadows has no late night arrivals any more I think the warden might have had the right to refuse entry it now has new barriers and has had from January 2017

I did as stated, called in advance and they told us that 20:30hrs was ok, everyone seems to be missing the point, the service was a disgrace, I pay to be a member of the supposed best club but was subjected to an inquisition on the phone and in person, they should have cancelled our pitch, I’d have had a better night at Watford Gap services!!

Perhaps you should contact the Club's Sites Dept, BD.

Maybe not in the rules but it has become an accepted practice I suspect to hold the card of late arrivals and settle the next day rather than the warden opening up the office and firing up the computer. Doubtless to save he or she from eating into more of their evening leisure time. 

I’m currently waiting on a response.