I think I did something reasonable, but I'm wondering.
I exported a fairly hi-poly mesh (1098 tris) from Blender (subsurf applied and decimated).
I exported a separate lo-poly one (28 tris) to use as a mesh collider.
Pictures below. Is this a normal procedure?
Yes that would be recommended if you wanted to create a low poly mesh collider. I typically wouldn't recommend mesh colliders unless you absolutely needed to use them for precision.