Hi Monacot,
you have my sympathies. With 2 children and bags you have no idea what to leave behind, and that 50 yards may be too long for anyone's comfort if the baby is left in the car. (I had instances when both my baby and the toddler were asleep)
Now I am not sure if you have a garage at the end of the drive like we do, but whenever I see someone parking across it whose car is not known to me I ring the police (non emergency number). I complain that we have a motorbike in the garage and that we need to use it. In most cases police will manage to ring the registered owner and they will move the car. I guess call from the police will stop them parking in front of my driveway. (For what is worth)