Roger R wrote:

The lighting circuit is on one fuse. There are two line cables and two
neutral cables heading off, presumably a pair for each floor.

Not the question you asked (which Christian has answered fully), but
should you/they ever be temped to separate this into two circuits
(upstairs and downstairs) be very careful you don't create a 'borrowed
neutral' situation where one or more lamps gets live from one circuit
and neutral from the other. With the wiring system in question it's
highly likely that 2-way switched sub-circuits for the stairs and
landing lights take their live feed from one half and return the neutral
to the other. If this is the case it means you can't do the split
without re-wring the hall/landing lights.

And what's that piece of flex doing connected to the lighting fuse?

--
Andy