Hi Graeme,
It’s true that simple collision shapes must be convex.
There’s a distinction between ‘simple collision’ (simplified proxy mesh to be used with physics/collision system) and ‘complex collision’ (using the mesh geometry for collision)
I’d start by removing any simple collision that might have been generated on import. (Note my screenshot does not have any green simple collision shapes)
And set the mesh to use ‘complex as simple’
(This box was modeled using the built-in modeling tools)