The details are:
Barnet London Borough
Parked in a special enforcement area adjacent to a footway, cycle track or verge lowered to meet the level of the carriageway
Shirehall Lane / Hendon Park
I think there could be grounds to challenge, as the PCN online (image 9) states "No waiting", which is not true. There were no signs or plates informing about waiting or parking restrictions in that area.
Also the car was parked away from the flat part of the dropped kerb, but was close to the angled part going down.
And the ground was dirty at the point the road and pavement merged, so it was difficult to see what was what.
This is clearly designed to try and catch people out as other sections of the same road had double yellow lines, but there were no road markings or plates indicating parking wasn't allowed here. The dropped kerb would allow cyclists or e-scooters to mount the pavement and cycle into Hendon Park - something that isn't even allowed anyway. There is a post in the middle of the path to stop larger vehicles entering the park.
Finally, the offer to pay half price has disappeared - hopefully not a policy change.
Photo links - images 1-8 are the Notice To Owner and 9 is the online 'evidence'.:
https://imgur.com/a/tkBsSva