Thanks everyone for your super quick replies.

John, I've had a look at the switch downstairs (which also has 3 cables
running into it.) This switch has a connector in it with 6 holes
(sorry about the terminology!) There are 2 commons. Both of these
have a red, but the bottom one has a loop of red (independant of
cables) going from the common to the 1 way switch hole in the centre,
which has a blue wire going into it. By the way, one of the cables has
a yellow wire - no blacks in sight.

Unfortunately I don't own a multimeter, but I don't mind working with
electricity. (turn the mains off, and when I turn it back on, switch
the light on with a broomstick! ha ha)

Thanks for your help!