Ok - I think I have figured it out, so I will explain what I did in case anyone else is facing this problem.

I ended up looking under skin>edit smooth skin>add influence and then picked another one of his skirt slats which were behaving correctly. That gave me more options in the influence window to paint weights on the offending polygon. You only seem to be able to add other polygons as influence objects rather than other joints.

Not sure if this is the correct solution...