Thank you, xeen, for the information on using relations for bus routes. I just wish that all this stuff was in one place.
It appears that I have to do each variant of a bus route as a separate relation. This seems very cumbersome: one of the routes here has six different opt-outs (same spine of route, but serves different roads/villages/estates at different times of the day) and all of them are possible. Each trip seems to be different; am I really expected to create 64 separate relations (albeit grouped together under route_master) ?
Am I supposed to create a separate relation for a route whose "opposite direction" is a simple reversal of the "forward direction" ? What if the "reverse" route differs only by the path it takes round a roundabout ?