Thanks for the help Michael, the solid is as you say shellable. I am taking a closer look to see what you did to it.

Incidentally, I find I am switching Object and Straight snap on and off a lot. I toggle Grid snap with the g key and:

script:moi.drawingAids.gridSnap = !moi.drawingAids.gridSnap;

...but replacing gridSnap with objectSnap (a guess on my part) linked to the o doesn't seem to work.
What are the method names I should be calling for Object and Straight snap?

Cheers
Dan