Just a note to say many thanks to Dave who has sorted the problem out. For completeness of this thread, it wasn't anything I was doing wrong per se, but a feature of Sketchup that meant because I was working at such a small size (the knurled nut was around 7mm square), SU couldn't make the small faces that were a result of the arc being lathed. Upscaling the entity and completing the operation and then downscaling got around the problem. Thanks again Dave.
Cheers
Gidon