From the GSV photo. I would think the property line is the scrubby hedge and trees. Of course, you can take them all the way to the Traffic Penalties Tribunal, to test the matter. To do this, you must forego the discount. In addition, you would have to wait for the Notice to Owner, and submit reps again and be refused before you can register an appeal with the tribunal.
Are you the Owner and holding the V5C Registration Certificate for the vehicle, with name and address correct ?