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 rose

1 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.