This complex shape has been built up by joining surfaces. I've had success in the past composing this sort of thing into a usable solid, but not here - there's obviously something stopping that from happening.
Question: is there some tool or script available that will highlight edges/surfaces causing the problem? ( I mean short of zooming in close on every edge and inspecting it 'manually')

I should add I'm using v3 - and yes I've simplified the shape as much as I can ;)

Hi Brendan, yes you can set up a script on a shortcut key that will highlight naked edges, see here:

http://moi3d.com/forum/index.php?webtag=MOI&msg=6051.2

If you can e-mail the .3dm file to me at moi@moi3d.com I can try to help you get it closed.

- Michael

As always, many thanks Michael!
I'll run the script & email if I'm still scratching my noggin :)