I should have known really, given the creative cable routing shown by however rewired this house in the mid 60's (AFAIK)......
Rewiring the dining room, redoing the wall lights (some new positions, and anyway, no earth in the circuit). The feed and the switch drop were underneath the floor, but I couldn't find where the cables to lights went at all. gave up disconnected the mains from the circuit and got on with the job.
When installing new cables I found the old ones. They had been run around the room in the plaster where the old picture rail had been (not even in line with the lamps - these were lower. Presumably, this was a handy slot in the wall when the rail was removed.