Alright, in playing around some more, i have ensured all toplogy is clean and welded where needed.
all normals are facing the correct outside orientation,
the remaining hole is facinating in that it only appears inside the Static Mesh editor
where prior, without High Precision Tangent Basis enabled, the holes would be present inside the level.
with high precision tangent basis enabled, the hole goes away inside the level
in static mesh preview window
in level with the High precision tangent basis enabled
