Hi, I have been in exactly the same position as you. I have a private parking space which is clearly marked as private and have had someone parka 4X4 in it on a Saturday afternoon and it was still there the next morning. So I phoned the police and they told me that I was quite within my rights to have the car towed away. But they said if I gave them the registration number they would have a look on their database and see if they had an record of the owner. They had, and they called him for me and he was outside my flat removing the car within 10 mins of my phone call. So yes, in the first instance call the police as that is the easiest option.