Tom, Thanks for this. This is one of the better interpretations I have seen; Where there is a requirement for more than one device in an area, these radii must overlap to ensure that there are no blind spots. I have seen some sites that say the distance between detectors should be half of that stated in the BS to give the overlapping.
A query which has me puzzled on heat detector spacing. BS5839 gives spacing of 5.3m from any point for detectors on horizontal ceilings. However, it seems accepted that, within rooms, this should be halved to ensure overlap. Many manufacturers technical guides show this. I cannot see anything within the BS on this, and cannot see any circumstances where the 5.3m from any wall cannot be met . Any ideas?