I've looked a number of pages:

osm.wiki/Area
osm.wiki/Area/The_Future_of_Areas
landuse=*

and likely a lot more besides. From what I could tell there seemed to be some debate around the subject of areas in general. There seems to be two possible solutions (trying to avoid using "ways" in too many contexts!). The closed ways solution, which seems problematic when two areas have a common boundary, and the multipolygon solution which seemed to allow me to do what I was looking for. It is possible that I've missed something about closed ways that allows solving that particular issue neatly though. I'm sure we can fix it, if that is the case.

Steve.

Hi,

Apologies if I've done this the wrong way. That appeared to be the easiest way to model the boundaries that belong to multiple relations. The wiki appears to imply that is the best option for more complex areas, but maybe this is a case where it is not giving me the whole picture...

I could only work out how to use closed ways in the case that the boundary was not shared with other fields. Please let me know if there is a better explanation that I've missed somewhere!

Steve.