dear helpful peeps, I am very happy to declare I have fixed the problem.
Yes, it is a loose connection in the bathroom lights pull-cord switch.
I should have checked this at first point instead of went through all the hassle in the loft.
It must have been the 'bedroom-dimmer-controls-bathroom-downlight' as well as 'stair-ceiling-light-always-working' but 'the-rest-always-not-working-at-the-same-time' confused the hell of it.
So after two nights of 'meditation' I gathered my strength (you can stop pressing now) and tried a few different connection options at the junction box. There are 4 pairs of cable at the junction box: a -switch live - dimmer for the bedroom; b - bedroom ceiling; c feed to the bathroom (fan/light) ; d live in.
In one configuration, I managed to tried the permanent live/switch live messed up situation.
At the end I put back the connection to what it original was. And check the pull-cord. :-D
Unfortunately I can't share the photos or link to the photos for each of the configuration I tried.
But thank you all very much!
Very happy with myself.
(It turns out I cant post smilies neither)