like a bellend i saw a light fitting i really liked online and impulsive bought it. however ive now realised my ceiling hanst got a pendant rose but has in fact sunken spots.

how easy / cheap is it to install a ceiling rose connected up to the spots loop? without smashing holes in the plaster board?

If you can get to the wiring its very easy. If smashing holes in the ceiling is out then you will either need to lift the floor above or thread a wire from the new rose to one of the spots. This might be easy or really hard depending on the joists and other things being in the way.