Given James' employer, I can't see that overall approach being recommended

James... Try removing the screw under the front of the sill cover, then slide the whole thing forward after 2" it should simply fall off.
Then you can assess the damage properly.
To refit, simply slide the cover onto the rear end cap and tap it back onto the clips. If the front screw doesn't quite locate then slide the sill cover slightly to correct the position and refit the screw.
If you have a dog, bag up tye poo in nappy bags and collect them in a a box in the garage. Double bagged it won't smell. Then empty it in to your rubbish bin the day before bin day. He will soon get pissed off at finding bags of shite every time he checks your bin.. You could also move your bins to inside the garage. That way if he wants to check them he has to tresspass to do it. With a camera you can then build a reasonable case against him.
First issue is to deal with the car. But, as said earlier, do NOT let it fester. Your relationship at home is far more important than the one with your neighbour, and if he is this much of a nuisance after a few months, it will become serious problem rather than an irritation.