If all units are numbered at boxes in alva st an emergency vehicle could go right past. So maybe different street addresses would be good? not all alva. Emergency services do like someone to stand out on the road giving directions.
Is it still the case that no vehicles, including emergency services, can access the common ground behind the H and A blocks? So in most cases going to the letter boxes would be a waste of time.
Our brigade used to do recces on practice nights, if there was some change at an address we would ask if we could go and have a look around. find best access and water points and it made a change from the usual practice.
Regds roz