Thanks for your replay shaderbytes.
I think your description of light maps having so many splits and islands explains why I got a fractured panelled look in my geometry after the light bake in Unity - I could see lines that looked liked cracks on trees in my scene.
The reason I've done it this way is partly because of a reply by Glenatron on his Tiki Treasure where he confirms exporting a second set of UVs and doing the lighting in Unity because "the tools are better".
I've always wanted to build game assets and scenes, and Unity seemed a good entry level game engine. But yes, after my first results I am looking at options and considering how I would do this in Blender.
I've just seen a light-baking tutorial for Blender. My only concern here is maintaining a game look. I want it to look like something in a game, not a 3D render in Sketchfab.