Is there any reference in the FSO that you educated people could point me towards that - Pacifically says that separate blocks/properties require separate FRAs. I’ve encountered a housing association that will ask for one FRA for two separate addressable blocks which are semi detached but have their own entrances and exits with their own addresses.

If they are separate premises they should have separate FRAs, but if 1 premises is subdivided into separate wings/cores (e.g. own entrances & stairs) then they can be under a  single FRA.
Ultimately it's down to the Courts - some RP's try to save money by lumping entire estates into single assessments despite being unlikely to be unsuitable as they are too vague & general - however if that's the only issue and there are no other serious physical or management fire safety deficiencies even if technically wrong it's unlikely to score enough points in a fire service audit to trigger enforcement action