Hi Jack
I have a 1949 Light 15 which should be pretty similar to you car and the dashboard light switch in located in the bottom right hand corner of the dash. I use my switch for the wipers as the wiper switch is not reliable. I do not have a manual advance/retard mechanism and there is no hole in the dashboard to fit one and as the car is pretty original it may never have been fitted, but it is 70 years old so who knows.
Cheers
Richard
Richard Larter