So... I imported a simple seesaw from Maya (FBX). It has animation so I created an empty object and placed my seesaw inside it. Needing additional box colliders I duplicated it a couple of times and scaled and placed my colliders and everything was fine at runtime.

I then placed a NPC at each end and needing some support to hold them in place until attacked by my FPC I decided I needed a couple more and still it was ok but as soon I got up to six colliders (one original and five children in descending order) they don't seem to get along very well at runtime and sort of explode out from the center like a book dust jacket.

A cue, a nudge or a slap in the face would be greatly appreciated.