Well I dont know what the problem was, but the solution was pretty easy in the end. I lost count long ago of how many times I'd clicked on 'Remove Selected Object' and 'Add Selected Object' but to layers I created on the first day. All it took in the end was to 'Empty Layer' (every layer), delete them, created new layers, re-distributed the objects in the scene to where they have always been and everything behaved....?