You have the patience of a saint mate - really need to catch the individual in the act (CCTV etc), and keep informing the police every time, they have to do something in the end otherwise they will just say that you didnt report it. I know I would have lost the plot by now!
why not pour a kilo of sugar in his fuel tank? then watch as his engine doesnt start 
you can buy good quality dash cams from ebay
I don't really need to keep reporting it to the police, as if it'd the neighbour I'm 99% sure it is then I also have weekly visits for stupid complaints for random things like throwing lit fags and empty beer cans into his garden. The 2 quite big flaws with that complaint though is I don't do either. So I'm hoping eventually he'll make enough false complaint to then get to the point that they will ignore him completely or charge him with wasting time.
A few months back his other half's 15 year old focus wouldn't start, witch apparently due to the complaint I got was apparently my fault. Despite the fact that all modern fords you need the key for to open the bonnet, but then his own car isn't much better being a similarly aged Renault estate. Plus I was just thinking the other day, out of all the cars in the street the mini is at least twice as old as the rest and probably worth a dam sight more.
Anyway, I've found the old siren witch still works, to the alarm I had fitted a few years back before the immobiliser started playing up. So all I need to do now is wait until the weekend so I can pop round to a mates to refit iand wire it up to the boot and then see what happens with the boot left unlocked when I return.