The number of triangles makes no difference. I always have it set to about 500,000 and increasing them leaves it in the same state - only with more triangles... :-/
Anyway, overnight I re-ran the earlier set of photos that came out ok and it now looks like the later one with missing details and broken edges - so it looks like something has changed in the new build of PS.
Point cloud: 700,000 points
Dense: medium, moderate filter
Mesh: 600,000
Decimate: 500,000
I guess this problem stems from solving the texture problem of the previous build? If so, that's a shame. I'm reverting back and will run this through again. If it works I'll pass on this build - I'd rather work around the texture problem than have a mesh which is clearly not generating properly.