Hi barney, I've been looking at your problem here, and it appears to be some kind of bug in the fillet surface generation, something like the fillet cross-sections are rotating or wobbling somehow making for a badly shaped fillet.

I rely on the Solids++ geometry library that I license to calculate these fillets for me, so it tends to be a difficult area for me to fix myself, it probably will need to be sent to the authors of the geometry library for them to examine.

- Michael