We moved house in May - it's the end house down a short cut-de-sac just 15 houses away from where we used to live. This morning I received a text from a current neighbour to say that my ex-neighbour was outside our house at 7.20 this morning just looking at our house. If you saw him or his wife (she's normal compared to him) about would you say anything? I just find it weird and creepy that he did this and can't help but wonder if he's done it before.
I would also wait and see if it's repeated, it could have been something quite innocent like out walking and saw an interesting bird in your garden or something. That's something I might do without thinking.
He had an issue about parking, had the police out to us a number of times and (I strongly suspect it was him) we got sent a fake police letter about parking. He is widely regarded as a bit weird and he is very territorial.
no law against standing on the street. If you're thinking he's stalking you you'd need a longer list of examples, and even then they don't sound much like harassment.
I'd go by the James Bond rule....Once is happenstance, twice is co incidence, the third time its enemy action. If it continues to happen I'd do a 111 call and express your concern