OP
Octopus
Yes, 3 core for the interlink but ...
I am not an "expert" and don't want to be Mr know-it-all but; if you're running a separate way for the detectors then why RCD it? The danger is that it trips and you don't notice (and so loose your fire protection).
RCD is required for all LV circuits in the bathroom. So if you 12v the lights then you can run the smoke/heat detectors (with battery backup) from the light circuit without RCD.
That's my understanding (I may be wrong).
Laurie
As you say, you could well be wrong!