Hi KME, I'm glad it's not just me who can't get to the bottom of these regs:_|.
It seems if not HMO you can do almost anything you like, including battery only detectors in a 3 storey victorian property - local council considered this acceptable. Same council as soon as you hit HMO on similar property insist on full fire panel. It seems no one has definitive answers to your question, altough you could also take advice from loacal fire-officer to get a more practical view of things.
IMHO i would opt for the AICO radio link system to a panel (the panel is mostly to aid the fire brigade locate exact position of fire) - parts are dearer but massie saving on installation time, and flexible enough to re-work at later date if requirements change. Smokes on stairwells, heat inside flats as you describe - this gives good coverage of fire escape routes and places 'sounder' within each property. Within each flat, without knowing layout, a single centrally placed smoke is probably acceptable - same as mimimum requirement for re-wire.
The only other thing I would consider is emergency lighting on stairwells and possibly in entrance to each flat.
Hope this helps, its a very confusing subject.