Matt, i probably didn't explain correctly.
Most flats had one standalone smoke in the living room, heat possibly, in kitchen and a communal smoke adjacent to front door with sounder, sometimes break glass.
There was then smokes and bgs on communal landings.
I removed all communal gear completely, so no detection etc on landings or in flats.
Flats existing standalone stuff was modified to mains interlinked.
Prior to these mods, all flats had to be made 1 hour fire rated, attics, risers, electrical enclosures, extract ducts, cable routes, anything between floors or walls that could allow fire to migrate.