I follow this page for guidance on multipolygons :- " http://wiki.openstreetmap.org/wiki/Multipolygon_Examples " It indicates that the tags should be removed from the “outer way” and placed on the multipolygon itself. Also the large island which includes a car park should be made into a multipolygon, again the tags should be removed from the “way” and put on what will be a nested multipolygon, nested within the first multipolygon. Regards.
I read that and did as it advised: put the tags on the relation. The renderer is happy now, and the data has not been “bent” just to achieve good rendering. I’ll have to think through the car park: a) not sure what’s wrong with it at the moment, but b) it’s not really a car park IMO.