Oxford's ECNs are all illegal, because Oxford is a Civil Enforcement Area. If an ECN is not paid the only recourse is a criminal prosecution in the magistrates court, but
regulation 8 of The Civil Enforcement of Road Traffic Contraventions (Approved Devices, Charging Guidelines and General Provisions) (England) Regulations 2022 says:
8.—(1) No criminal proceedings may be instituted, and no fixed penalty notice may be issued, in respect of conduct constituting a parking contravention, except a pedestrian crossing contravention.
We've seen this sort of nonsense before, for instance on my home turf in Great Yarmouth:
https://www.greatyarmouthmercury.co.uk/news/24411882.police-falsely-accuse-man-illegal-parking-yarmouth/Of court Oxford City Council would never accept this, but I'll be happy to come to Oxford Magistrates' Court with you if this goes that far (and while I can't speak for him, I'm hopeful Ian would also come along).
The following wording would suffice at this stage:
Dear Oxford City Council,
The car park in question is within a civil enforcement area, pursuant to regulation 8 of The Civil Enforcement of Road Traffic Contraventions (Approved Devices, Charging Guidelines and General Provisions) (England) Regulations 2022 you are not entitled to issue any notice under the criminal law nor are you entitled to bring any criminal proceedings in respect of any conduct which constitutes a civil paring contravention. In any event I did pay for parking and my vehicle is of a class authorised to park in this car park, so your case really doesn't have legs in any event.
Yours faithfully,
That the car was not parked in a bay or whatever else doesn't really matter, they can only pursue the allegation on the face of the notice, pursuant to Article 6 of the ECHR which states:
3 Everyone charged with a criminal offence has the following minimum rights:
(a) to be informed promptly, in a language which he understands and in detail, of the nature and cause of the accusation against him;
So, they can only really pursue you for parking a vehicle of an unauthorised class, if the car park bylaws says your vehicle is an authorised class of vehicle they can't chop and change the allegation and later say your vehicle was authorised but parked in the wrong place. If their silly little data terminals don't have the right contravention wording for "parking in a restricted area in a car park" that's their problem, not yours.
@Nosy Parker flagging for your awareness