I've only ever seen switch wiring before that uses the same cable i.e.
live in (red) and live out (black with red sheath). The stuff in our
mid-80s house is different.

There are exceptions, but generally where there is one light there are
two cables. The reds are both used in the switch and the blacks
tidied into a choc box. Where there are two lights, there are three
cables, again with only the reds used in the switch and the blacks in
a choc box.

Would I be right in thinking that those black wires are likely not
connected to anything at the other end? Or might they be neutrals?

Given that nice pattern of light fittings+1 = cables in switch, does
anyone know why our living room, which has 2 wall lights, has four
cables in it (same setup as the others though - one red in, three out,
all the blacks in a choc box)? There are no other switches for that
room or, as far as I can tell, any other electrical sockets/lights to
account for the extra wire.