Surely the letter from the solicitor is harassment enough. I think that alone deserves a call to the police.
However it may be hard to PROVE your other accusations if the neighbours deny it.
Of course it may be that the police think YOU are being paranoid, by accusing neighbours of watching you and taking notes (I am not saying you ARE paranoid, but the police could consider that you are).
Starting your car and going back into your house it not the most sensible thing to do, and I think leaving an unattended car with the engine running is against the law, but that hardly sounds like something the neighbours should be too concerned about.
I bet they are at home all day and have nothing to do. We had an old couple who lived next door to us and the husband was a bit like that.
Once a taxi driver parked in our road to have a sandwich and take a break, and my neighbour went out and told him he was not allowed to do that and that he should move on.
I think SOME older people get like that.