Typical ... having built a new house found I need some cable where I
didn't allow for it ...

Thinking of putting in Solar pv panels so need to get feed in cable from
loft down to consumer unit.
I can drop down to garage - so can easily run this across boarded face
of garage ceiling ...
I also intend running in 2 extra lengths of CT100 (and multi core DC
cable) to go to outbuildings for CCTV ... which I could also route
across garage ceiling.

(Ceiling is boarded, on counter battens, fixed to silent floor joists,
think it would be very difficult to try & feed cables above the board. )

I have never mixed power & non-power in same trunking ... what is the
ruling ?

Can you get small trunking ( eg 25 x 38 wide) which has a centre rib
separating halves ?

I could run twin 20mm conduits .. just thought a single trunking be easier