Kerwin, thanks for the suggestions. I've already looked at capsule zones as an alternative and they were free from this effect but didn't quite give the result desired. The item already has a huge number of additional bones as well (48), so I'm reluctant to multiply this any further.

Considering that the issue occurs only along the centre line between left and right halves, however, I've tweaked the original model to remove that line and re-imported and rigged the figure and - hey presto - the issue no longer occurs (see new image).

As it doesn't happen with capsule zones when a centre line exists, or not with spherical zones when no centre line exists this seems to suggest a problem with spherical zones used in conjunction with vertices that sit on this 'join'.