In a general needs property converted into flats with a communal fire alarm system (linked to heat detectors within the flats), a simultaneous evacuation strategy and no staff on site, is there a requirement to identify a specific assembly point on the fire action notice?

Ideally yes so the residents are all in one place to identify which flat they are from and pass info to the fire service and to stop them congregating in the entrance and getting in the fire services way although they may well ignore it!