Thanks for the help. I have had the battery disconnected for 3 days, but in my case reconnecting didn't solve the problem
I don't think its the light switch because when the courtesy lights are off (driving lights on) pulling the light switch out works as expected and the action seems nice and firm. I do think it is related to the central locking. When I bought the car one of the declared faults was that front passenger side central locking didn't work, but it did (!), for a while, then it stopped again. If these lights go out after 30 mins, presumeably when the ECU goes to sleep (?), the battery drain should be quite small, so I'm just going to leave fuse 7 in and see what happens. Maybe I'll try slamming the drivers door a dozen times - but who knows whatelse may stop working!
All the best,
Steve