I would really like some opinions on whether I am being petty here, I'm wondering if I've lost all perspective. Also I know there should be a diagram, my ds has hidden every pen in the house so all I can do is apologise 
I live in an estate with alocated parking. I live in a cul de sac with three houses in a row, each with one allocated parking space. I don't drive but I do have family/friends over several times a week who obviously utilise my parking space. This means that my space is empty more often than not. I have no issue with residences guests using it every so often, I expect that but there is one neighbour who basically takes the piss. This neighbour is coincidentally the only one who has an allocated drive so never has the problem of people parking in their space.
They have 2 cars so can't fit them both on the same space. They are both big cars, 4 door big things/large family style cars. When I moved in they always parked one car in my space meaning that I had to go round and ask them to move everytime I had a guest round. This carried on a while until I had enough and asked them to refrain from parking there at all. They got a bit huffy (I suspect they don't see the problem because it's empty most of the time and far more convenient for them to use it) but did stop. For a bit. It's now the same old routine. They stop for a few months, then start again. I ask them not to, they don't for a few months and then start again.
It is driving me mental. I find it so rude. They literally use it as their second car parking space. I am sick of the sight of the bloody green thing in my space all the time! So aibu to be annoyed? I don't see why I should have to knock on the door and hope they are in and haven't gone out in the other car when I need to have access to my space. But I'm fed up of asking them! Half of me wants to leave it because I can't stand the hastle and the other half once to tell them in no uncertain terms to pack it in!