Thanks for the advice guys. I guess you intuitively know what to avoid doing after a while, and the other methods of achieving the same result - in this case - are probably faster anyway.
However, having dabbled (emphasise 'dabbled' ! )with coding 3d engines in OpenGL, I still can't find any good reason why the problem should happen in this simple case. At a stretch, I could understand some normals getting flipped, but this just seems like a good ol' bug! (...but I'd love to find out what I'm probably missing.)


Still, who cares... superb software; I can't wait to get better at it, and this seems one of the friendliest, most clued-up forums I've seen :-).

Thanks

Steve