Another wiring question!

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I'm in the process of replacing a ceiling batten light with an LED tube light, and things are not as straightforward as I'd predicted (to me anyway).

There's a simple setup of a switch and the light, however the switch cable hasn't been sleeved or marked, and I don't have a meter tool.

Here's what the wiring was like on the original light:



and here's where I currently am - just the two joined reds connected.



The wiring block on the new light is Live, Earth, Neutral.
How would I go about identifying the switched live black wire? Presumably that would also connect to L, and the other black to N?

Thanks in advance for help.

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Yes, but one black is the switched live (from the light switch). I don't know which.

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Yes, I'll be sorting that out too!