Yes that's resolved it. You were right about the possible absence of a .mtl with the original .obj

when I import the .obj to blender the materials are already there. I can immediately export with no changes which creates a .obj and .mtl file. Then when I import into sweet home 3d the materials will ...

Thanks for the help. I'll download blender and give it a go.

I've got a few models that apply colours or textures to the whole model. I'm looking for a way to label parts of the model to be coloured individually. So that a handle could be a different colour to the rest of a cabinet, for instance.

But I don't know what software I'd need or what this would be ...