One Way Switch Wiring Issue. Help!!

Hi all,

I'm hoping someone on here can shed some light on an issue I currently have with a bedroom light switch.

Yesterday evening, I attempted to change the standard light switch in one of the spare bedrooms for a dimmer switch. I initially swapped each wire, like for like, so that the COM's wire from the old switch was put into the COMs terminal of the new dimmer switch and so forth.

With the electricity turned back on, everything seemed fine, as the dimmer switch was working as intended, however, I soon discovered that the lights in my bedroom, en-suite and walk in wardrobe were acting up, as they'd either fail to turn on or began to flicker when they did work etc. Unsure what might be causing this issue, I decided to cut my losses and just refit the old switch, believing everything would go back to as it was before I had decided to meddle with it.

It was at this point that I realised that the L2 terminal had been removed (see photo) from the original switch, which would probably make sense, as it is just a 1 way switch, but what I'm now wondering is what the third wire is for? Where should it go? It was originally just pushed into where the L2 terminal would be. Despite this, I refitted the switch as it was originally and the light is working fine, but now none of the lights in my bedroom are working. If I swap the wire in L1 terminal for the one that's sitting in the empty L2 terminal, the switch no longer works in the spare room, but turns on the lights in my bedroom. I'm totally at a loss as to why the switch will no longer work as it had originally.

If anyone has any idea how I can rectify the issue and if possible, install the new dimmer switch, I would be eternally grateful :) Failing that, I'll settle for leaving the old switch on the wall.

Cheers