OK thanks good tip.... So I've tried the A360 online viewer which threw up all sorts of crazy things so I downloaded one and it looks better, however...
The balustrade I mentioned on one model is still in the wrong place as it is in ArchiCAD. On the other model, where all sorts of funny things are going on, there are still plenty of issues but not all the same ones as I'm seeing in ArchiCAD!
Some of the problems are seemingly resolved - the mystery wall (which was actually an extension of a wall, not a separate entity) isn't there, some decorative detailing which had gone a bit crazy looks right, and some missing windows have returned (but not all!)
However some of the problems are still there such as my missing door and windows buried inside walls, and there are also some other entirely new problems. A couple of random bits of wall are transparent, for example....
There is no logical pattern to it - there's a window family type, for example, of which I have three when there should be four. So it doesn't seem to be attributable to particular objects!
So an export problem, or a bit of both?