I’ve tried everything that seems sensible to me but can’t figure this out, can anyone help?

There was a single switch for the room light, plus a separate wall light wired in to a socket so it was on a live cable constantly. I have routed a new cable from this wall light to a new double switch that will operate both the wall and main light.

So, the wall light cable isn’t live, it runs from the light to the switch. The ceiling light cable is live.

How the hell do I wire these up? Every way I try it ends up with the wall light constantly live, and the ceiling light working fine on and off at the switch.

Photo; left hand cable is the wall light, right is the ceiling.

Diagram of how you’ve wired the new cables might help.

If the room light works ignore it, it’s 2 separate switched, yes?

Just lick the wires.

Post video.

It’s a bit difficult to be certain from the picture but I think it won’t work because you’ve not got a neutral at the light switch.
The right hand cable is probably brown=live black=switched live.
The left hand cable is probably brown=live at light black=neutral at light.
A bit uncertain why it’s run in 3 core with the grey taped up. Can you clarify?
Rich

If I’m right, you’d need to run the cable from the light to a junction box in the ceiling (make sure its maintenance free or accessible for mainetance purposes). Then, the junction box needs to be fed with a permanent live neutral and earth from the existing lighting circuit. Then, run a cable from the junction box to the light switch. This will be live down/switched live back. Make all connections in the junction box and it will work.
Rich.

As above Dan. Lighting circuits are complicated. In the circuit junction box (attic?) you will see the black switched live (from the switch join) onto a red/brown live to the light fitting; a black/blue neutral comes back from the fitting to join the neutral bundle.
You need a neutral in the switch (don’t be tempted to use the earth instead!).