Advice Needed After Changing Ceiling Lights now Stairs Light not Coming on Properly

[Nads75]

I have put new ceiling lights up today in the front and back rooms. Faced with nine wires in the ceiling. three red, three black (one with a red sheath) and three earth. I googled and found that the three red should be bundled and seperated off together. I fixed the earths to the light fitting, the two black to the neutral fitting and the black with red sheath to the live fitting.
Electric back on and all worked fine.

Until tonight when i took my little one up to bed. I noticed the bottom of the stairs light did not work? Turns out the lights do work but only if I turn the living room light on and then both lights work at half power (dimmed lights) I can turn the living room light off using the passage switch! What on earth have i done wrong? See vid showing current set up.https://youtu.be/pp_s0iLvGbA

[ericmark]

Switch line and neutral to next lamp swapped in error.