Simplistically, am I right in understanding that a lighting circuit is
linear (not a ring main)? All the action takes place on the red (*)
wire. In each room there are two drop cables: one to the switch and
one to the light fitting. The electricity goes from the ceiling to
the switch then back to the light fitting (via the black) then through
the light bulb to the main black wire. The same is repeated in each
room.

This means only one cable is connected to the consumer unit. I
believe a red sheath should be fitted to the black wire at the switch.

(*) Although I refer to red and black, I appreciate these may be
brown and blue. I have ignored the earth but I assume every earth
should be connected to every other earth at every opportunity.