I am afraid, as said, if there are no parking restrictions in force (i.e residents parking permits etc) and your neighbours' cars are insured, taxed and MOT'ed you haven't got a leg to stand on.
Mediation maybe an option if the neighbour is a tw4t.
And as for seeing your car, one must be realistic. If you see some chavs stealing your car are you really going to tackle them? Would you risk being stabbed in the eye with a screwdriver (all car thieves will have one with them) for sake of an insurance claim??
Don't get me wrong I am all in favour of citizen rights against the scum, but I am a trained fighter, big, strong, fearless with a cool head and a working knowledge of how I could escape a prosecution when I plough the aforementioned screwdriver in the thief's genitals. But do you have these (maybe undesirable) traits?