I'll try to set the scene and avoid drip feeding.
Our road is a real mish mash of houses built at different times. The more modern have cobbles to the road edge. We are one of four that were built much earlier. Our four houses have grass verges in front but we each own the land up to the kerb. Two houses have no wall and have incorporated the land into their own. Next door has a wall and a verge. We have a wall and a parking bay - put in by the people we bought from and access by the original dropped kerb to our driveway. There is no pavement on our side of the road all the way along.
500 yards down our road is a long stretch of walled garden with parking for maybe 10 cars alongside.
New people moved in opposite last year - two cars, no parking. The day they moved in I came home and their removal van was parked in our parking bay. Fair enough, moving is hideous so I didn't say anything.
Then they started parking in our parking bay regularly. I went over and explained that we owned the bay and she was really snippy and said "actually roadside parking on a public road is for anyone to use (maybe she reads Mumsnet 
). So I explained that we owned it. This was just after Christmas.
Got home last night - torrential rain, hideous wind and she's parked in my space. So I went over and asked her to move. She huffed and puffed and said "I've had a hard day at work and didn't want to get wet" 
She then went on to say she'd continue to park in it when we weren't using it. I asked her how she supposed she knew my timetable.....
So how do I deal with this now?