/diy/ - Do It Yourself

Hi folks, and idea where the blue wire should go?

I've never seen anything like this before - old UK wiring and I'm young af.

The left hand switch controls one set of lights (set 1)
The right hand switch controls another set of lights (set 2) that are also controlled by another switch.

When I connect it to right hand L1 the two switch positions mean either set 1 or set 2 are on.

When I connect it to the right hand L2, it turns both sets on or off depending on the position of the right hand switch.

All other wires are connected exactly as when I removed it, though I did move them a little for this picture before I changed over the wall plate.

Very much appreciated if somebody can help - I'm having difficulty reasoning this out.