OP, the NOR you've posted allowed payment of £90 - effectively the discount - until 14 days beginning the date of the letter, 8 April. This expired on 21st so you're in for the full penalty or worse if you don't register an appeal in time.
IMO, the thread has missed a key aspect of your account which has not been examined.
in December’23 I drove to T3 Heathrow to drop off my kids and returned a week later to collect them.
My husband and I used Heathrow T5 to travel in March ‘24. We used the Meet and Greet parking facility and returned a week later.
A few days after we arrived home I received a PCN for the day we got to the airport. It was an immediate £90 fine. A week later I got a second one for the day we got home
Why didn’t I get any penalty notices in December? Same vehicle, same trip.
Exactly. Unless the limits changed between Dec. and March or you did not enter the ULEZ area (route to T3 being different from T5) but as 'The ULEZ was expanded across all London boroughs on 29 August 2023.', that's not likely to be the case, then IMO you had a legitimate expectation that your vehicle was compliant and not as you posted 'It’s a Nissan Navara so I’d thought it wouldn’t be compliant.'.
- unless your registered keeper address has changed since Dec. '23 and there are two more PCNs in circulation!