ring fire brigade, say you think there are people still in the house, and the fire alarm is going
mention how its one of those new fangled carbon monoxide ones.
I would phone the police, there should be a non-emergency number available somewhere online to get in touch with your local station
That is something you should never do.
No matter what time of the day it is, if a alarm had been going off for a specific period of time it's classed as a noise nuisance no matter what time it is.
Give your councils noise team a call as they'd have more powers then the police would in certain circumstances and they can call someone out to shut off the external alarm box (it's amazing how much you can learn from certain programs the BBC broadcasts during the day).
I would phone the police, there should be a non-emergency number available somewhere online to get in touch with your local station