Ah, thank you!
Yes the mesh repair tool was saying there were several extremely short edges!
The next thing I wanted to do was boolean difference the result with some spheres and realised also that when vertices are nearly coincidental the boolean fails but just by remeshing to keep the vertices apart the boolean works.
Good tips on mesh booleans!
It looks like either testing for coincident vertices or aligning vertices after the boolean if the result is open could be integrated into the mesh boolean operations to improve Mesh Booleans?