Quote - Is the problem
a) My modelling
b) Crossroads
c) Both of the above
d) Something weird
Any ideas from you lovely people?

We can rule out a) and c) as the model looks alright when imported as COB.
The most likely problem is that Crossroads doesn't understand something in the COB file.
If there is an option to freeze/convert/bake to polygons before exporting from Metasequoia I'd try that first.

What other formats will Metasequoia export too ?
There may one that Crossroads can do a better job of converting to OBJ.

Or if you have Bryce 6, you could export the OBJ file from it, as it appears to be able to read the COB properly.