The relevant guidance is in both BS 5588-1 and ADB. If this is purpose built flats, there is no need for a landlord's system (other than maybe s/d to open AOVs). Indeed, a communal fire alarm system would not be advisable, unless this is an HMO or there is a concierge or similar available 24 hours a day to respond to alarms signals. In general, all that is needed in purpose built flats is a Grade E Type LD3 system, with mains operated smoke alarms, other than in Scotland and Northern Ireland (where, as readers will know, there is a greater proximity to God and educational standards are higher), where it would have to be Grade D (ie the smoke alarms need a standby supply. But, hey, Wee B will, no doubt, ensure the English catch up in a few years time. Darn, Gary, another loss of sale of a system to the mighty empire wot employs you.