Hi Phil, also in some areas you've got some little tiny slivery surface fragments, possibly some remnant of some pieces being misaligned a little bit when cut with one another.

Here I've hidden all curves so only the solid and its edge are being displayed - it's subtle but notice that this area here looks a bit dark:



The edges are slightly dark there because there are 2 edges stacked up on top of each other there, there's a little squashed thin extra surface sandwiched in there, if I delete the big 2 adjacent surfaces there is this little weird piece left:



These kinds of little tiny squashed surface fragments will really mess up edge-based filleting. There is another kind of filleting which is surface-to-surface filleting, which is when you select 2 individual surfaces and run the Fillet command rather than selecting edges to fillet. That can sometimes be helpful for getting fillet surfaces to work with in areas where edges are messed up.

- Michael