PCN issued in residential permit-only car park

I am the leaseholder for a flat with an unallocated parking space. I have recently been letting the parking space out for a decent sum, since I no longer own the car. Today the tenant has received a Parking Charge Notice from the building. The reason is "not authorised", other relevant options being "No Permit," "Not In Bay," "Obstructive Parking," "Parked in Disabled Bay."

To be clear, the tenant uses and displays a valid permit (which was displayed today, photos held as evidence) and signage in the car park is very clear that only cars with valid permits are permitted to use the car park. It does not make any reference to being "authorised" or that registration numbers should be lodged with the property management.

I should point out that recently it has come to light that the terms of our leases do not permit the letting of spaces, and accordingly notice has been given to my tenant who is vacating shortly. This is clearly though a separate issue to the PCN.

My view is, notwithstanding my own indiscretion in letting the space out, the PCN is unenforcable because the terms of the contract as per the signage (valid permits only) have been met. Therefore there hasn't been a breach of contract and so I don't think they have a leg to stand on.

Do other forumites agree? Very grateful for any advice - I have read the residential permit thread but I think this is more geared towards residents who might be unloading/loading such that a fine would be unreasonable.

I have in the first instance contacted the property management to say the charge is not enforceable but think I should go to the parking firm. I believe they are an IPC member.