The black 'neutral' you can see is the live return to the light. It isn't 'the' neutral. When the switch is 'on' the black wire becomes live. (I assume you have black and red cabling as typical in the UK in the 1970's. - It should have a red sleeve at both the switch and rose). https://www.flameport.com/electric/lighting_circuits/lighting_ceiling_rose_other_colours.cs4