Dear Gethyn Lewis;
Please, could you precise a little bit more your problem? I think, you problem comes from the geometry definition or from the group definition. This is difficult to be sure without more precision about your simulation.
Besides, it is better if you post your problem directly on the spis forum:
http://dev.spis.org/archives/projects/spine/home/community/forumsPages/spis because more persons may help you.
Regards,
Benjamin