Thanks guys.
First: The drawing is a root drawing. Other drawings will access it when its complete. Its not an ex-ref. I do, however, draw solely using blocks and poly-lines. I edit all my blocks with the Reference Editor - not Block Editor. And I often copy blocks from other drawings. Maybe one of my blocks is infecting my drawings.
Two: I moved the origin and made all co-ord's positive. Still no joy.
Three: Regen made no difference.
Four: If the lines were in 3D space, then trim would not work either, and that is fine. Its something to do with the make-up of the construction-line or the trim commands in fillet and chamfer.
Have you all managed to fillet the lines. If so, then the problem is with my machine. Please let me know. As I say, I have experienced this on all the machines I have worked on with many different versions. Its definitely something real.
Any idea of the commands pertaining to trimming and construction lines. I am wondering if I or my cyber-friend, Jeff, is setting some parameter accidentally.
Cheers