if this is the only light controlled by the switch then you need to do as I said, turn the power off, put the meter on continuity and see which cable completes the circuit when you turn the switch to the 'On' position.
From there you can follow the diagram on the back of the rose
I don't have a Metre but the diagram I cannot see clearly and yes it the only light on the switch in the bedroom
apolgies I misread your post. But as riveralt says.
no problems , it baffled me had to get a friend who a electrician took him a while but who ever wired in the roof had some wires back to front but all done now thanks for all your help