We didn't know what the property was from the OP.
 
	Yes, I have serviced many a fire alarm in domestic properties, however all have been detached properties of various sizes.
 
	 
 
	Mr. H hits the nail on the head as usual, should really be mains interlinked.
 
	Mains interlinked would be installed with 3C&E mains cable.
 
	 
 
	Would use Alarmsense over Twinflex, DD.