The same process as polishing paintwork .
Use a compound paste with a polishing machine, comes out perfect
Deeper scratches may need working through different paper grades ending up with a 1200grade .
A local body shop can do it
Sometimes, it's 'marks' from the hedge wood/sap etc, rather than a deep scratch. Not sure how bad it is from your description.
You could try a weak solution of toothpaste (wet rag plus toothpaste) or car polish (on a wet rag). Try a TINY area first and polish off to see how it goes. Work along the 'scratch' rather than making circular swirls.
Really sorry to hear about it all though. Better that, than him smashing your mirror or the bodywork I guess? What a git eh?
Ive got my eye on this stuff its basically a polish designed for acrylic windows as ive got scratches aswell problem is i dont like using any king=d of cutting compound, only a few weeks ago I took some paint off the wing attempting to cut n polish the wagon having seen & spoke to the guys that done chris's motor @ Newbury.