Hi, I have installed two interlinked fire alarms as part of a renovation
project. However they are not linked at present just have two white
cables,
coming from each one. Is it ok to take a single core out of some
1mm/1.5mm2
cable I have laying around as I cant seem to find a single core that's not
red or black, and I have three way cable with a different colour core
which
I could use.

No. I'd use the entire cable and ignore unused conductors, or relay with 3
core+E. I wouldn't want to run a single cable, except in conduit, as it only
has a single layer of insulation.

Christian.