The latest survey of Porchester Gardens — just another couple of days surveying before it should be completed — has unearthed more of these lead markers and, possibly, has answered what they are. Here are two pictures from today’s trip [1] [2]:

The second picture is, I think, the key to unlock this: they are more-ancient versions of the traditional grave-stone wall-plates used to indicate the presence of Fire Hydrants or Sluice Valves (sewage safety feature). So, the prize goes to EdLoach (commentator in Mysterious Marker #2).

Do we have any agreed method of mapping these markers?

PS
This is the only Fire Hydrant marker that I’ve found so far in Porchester Gardens. Which is a little worrying.