Swarm landed in Wall Cavity Yesterday- Anyway to coax them out?

I would be cutting the drum in half with a small angle grinder. Tape the cut together as you go to dissuade the girls from having a go at you. If they have been there a long while there should be plenty of honey.

I agree with @sciencemaster’s suggestion. I think the heat method is risky, as you may melt the comb and make a sticky slumping mess blocking the hole. Also, I am not sure that heat would convince them to move if they have brood inside the drum.