Hi barney, so scaling up like Jamie was talking about before does seem to improve the shape a lot in the area that you had highlighted in your screenshot.

Here's what the original looks like in that area when filleted with radius = 0.08:



Here's what that same area looks like when the model is scaled up by a factor of ten and done with a radius = 0.8 :




Still a hint of a problem on the bottom part, but a lot better.


But it definitely looks to me like a fillet surface generation problem in the geometry library, I don't see anything that you have done bad here.

- Michael