How are you sure these vehicles are not taxed and insured?
These are private parking spaces and the police will not get involved. Vehicles do not need insured to be parked in such places and they may have SORN if they are kept off road.
The police will only be interested if the vehicles are not taxed or insured and are on the public highway. They will not be bothered about your parking issue.
As you have a tenancy agreement - is the neighbour also renting?



