opps forgot to say yes im lucky in that the loft hatch is in B/R and it's only arm lenght to bulb so I deffo know red and black from bulb
Then it is likely to be neutrals
N in, from a working light maybe ** say N1
N out, to next room *************** say N2
N out, to bath light ************* sayN3
If you feel competent, or else do not attempt
You can prove the Neutrals
Isolate each time and leave unused blacks safe each time
With all 3 reds together, will save you having to touch the switch each time you reset, so will be safer.
Using BLACKS as above
N 1+ 2 +3 not connected both wont work or dim
N 2+3 connected both wont work or dim
N 1 + 2 connected, no bath light, other light works only
N 1 +3 connected no other light , bath light works only
N 1 + 2+ 3 All work ok job done, Join in a proper terminal block
If you can prove the wires then there will be no doubt
Finally
power off refit switch , two reds in the L1 ( Live In from bed poss and live out to bed , not working ), and the ,bath red in the com on its own .
Or reverse way round, if you want, as it is only a one way switch
Leave L2 empty!
(Using L1 or L2 in this case would not matter but do as above)
Let us know tomorrow how you get on