An invalid registration is probably not enough to invalidate the FPN.
Were there any street lamps? A 30 limit where there are no street lamps requires a traffic order. It would be worth getting a copy of that order to check and make sure they have implemented it correctly. Also if it was in Scotland and it is an A or B road with or without street lamps then a traffic order is required. (Did I get that right? The law is quite confusing! You can of course ask the council for a copy of the traffic order relating to the 30mph speed limit on xxx road and they'll tell you if there isn't one.)
A27 in Hampshire and A35 in Chideock, Dorset have both recently been found to have invalid traffic orders.
If there are no street lamps then the signage must also comply with the diagrams in
the TSRGD 2002. Sign
670. The signs may require lighting at night, but that's even more complicated.
If there are street lamps then the signs should still comply, but it might not matter.
Was this the opinion of a police officer corroborated by another police officer? Or the opinion of a police officer corroborated by a type approved speed registration device?