Do you need to post signs to have cars towed from assigned spaces?

in England yes.

if anyone towed my car and there was no warning that it was a restricted area, it would be very bad, just make sure they haven't got details of the cars plate.

Not sure about other places in the world, but here in the good ol' US of A, all you need to do is call a wrecker if the automobile is blocking your driveway or is in an obvious no parking zone. It would be wise to post signs to keep people from parking there. Most of the time, a police officer will come by and issue the car owner a citation for parking in a no parking zone.