Yes I should have marked the switch cable with some insulating tape rather than going 'I'll remember which one it is'.
Bedroom 1 - all ok.
Bedroom 2 - new ceiling rose installed. Switch operates normally, light goes on and off.
Bedroom 3 - switch operates normally, but when light comes on it's rather dim. It doesn't require other lights to be on.
All lives (red) are wired together, as are all the earths.
2 neutrals (black) are wired to the neutral block on the new ceiling rose1 switch live (black) wired to the live in the new ceiling rose.
I presume I've wired the wrong black to the live in the ceiling rose but would appreciate any thoughts - even the ones that say I'm a numpty for not marking the cables.