I don't think this is how it was wired originally but it did used to work as expected, so not sure.
After putting up the new fixture on the ceiling the new fixture (4 LED bulbs) works fine and the switches operate as expected, but the wall light (2 halogen bulbs) only emits a dim glow.
Any ideas why the wall light is so dimmed? I had wondered if it is because there are halogen and LED bulbs on the same circuit, or if the existing wiring conflicts with how I have now connected the new light fixture.
I'm planning to remove the wall light altogether anyway, so will maybe get a better idea once I've opened it up (can't do just now as it's high up and I don't have a ladder).
Often one of the blacks is not neutral but is line return from the switch, in theory it should have a red sleeve, in practice often not marked.
So all earth wires together. All red wires together in most cases unless there is a second light in the room.
Often three black wires, two connect together and to blue on drop flex, and one connects to brown on drop flex.
Typical mistake is all three blacks together and typical report is when I switch off lights the trip opens, what has happened in that case is a short circuit when switched on, so lights are actually working when switched off.
In your case still the blacks mixed up, but neutral it seems only connected to lamp you have been working on.
So you need power off, and test each cable with a meter of some sort, helps to have second person to flick the light switch, one cable the red and black will show connected and disconnected as you flick switch off/on etc.
Any attempt to find by guess work is likely to damage the switch, not short cuts, has to be tested.
I tried to set out how to test on this forum once before, and got banned for my troubles, some one decided my advice was dangerous, and then disappeared. However he is correct in a way, any person who has the intelligence to sort this out safely does not need step be step instructions, you know you have a black in wrong place, you know the odd one out goes to the switch, and if you with power off measure between each black and the reds, the black which shows open and closed circuit as you flick the switch is the odd one out which you should mark with red tape and that is the switched line.
If you can't work it out, then time to call an electrician, there is a time to say stop, and get some one who knows what they are doing, but I think from your replies you can work it out.
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