I've been on the piste for a while, so joining this late.
Firstly I would pick up on one minor point.
Someone previously said;
....I'm sure you are aware that 7.5M is a number associated with the distance of the detector from any point in a room for life protection......
I believe that this distance is also applicable to property protection.
Secondly, someone previously mentioned old CFOA recommendations. Surely, these don't overide current British Standards? If they have any authority then I also demand that the WROT (Wiz's Rule Of Thumb) recommendations are also taken into consideration in the future!
In respect of the original question, BS7273 (spit!) almost certainly has some bearing on the issue. This BS can vary the automatic detector positions and spacings (including those in adjacent rooms) dependent on system category designation.
Does anyone know the 5839 system category designation and the 7273 (spit!) actuation category designation of the system in question, so that the specific recommendations can be ascertained?