jayaldous: Thanks Bung, that clarifies this really well. Behaviours really didn't match expectations based on the simplified one.
I _do_ keep left, and I don't drive in the right lane simply for the fun of it. I was aware of that statement, but I guess over time that thought had been diminished by observing otherwise every day.
Good to hear, your comment saying "... the motorway was just for driving in any lane unless specifically signposted." gave me the wrong impression.