Thanks TIG. Your suggestion re' extraneous bits of geometry that are "miles away" was the first (and only ) thing that entered my mind. Just about my only redeeming feature when it comes to using SU is my fastidiousness - I hate having untidy drawings so I'm usually OK when it comes to random fragments causing problems, oh and I'm forever clicking on "Zoom extents" to make sure .
I often import .dxf files, and though they can make things a bit messy, the drawing I had the issues with did not have anything imported into it at all.