Thank you Pieren - there was certainly an error there - part of an
island had been made a inner member of the relation, as well as the
whole island. I have corrected this, but the rendering issue seems
to be a bit more fundamental.
Richard Fairhurst, thanks for the explanation of why Potlatch 2 does
not render this. Perhaps it will be resolved one day! At least I am
aware of it now.