In a nutshell:
(1) To the shop owner: Enforce a fire resisting ceiling to a 60 minutes standard using article 8. (Measures to reduce risk of the spread of fire)
(2) To the owner of the bedsits: Enforce nice loud detection in the shared escape routes from the bedsits. (Or leave it to housing, depending on any protocols you might have.)
To go on about it a bit....
The 30 mins ceiling with linked detection to the commercial part is viable depending on how happy you are with it, but I tend to steer away from that where possible. My reasoning is as follows: From the point of view of the shop owner, you need to stop a fire in your premises affecting relevant persons. Although the people in the bedsits are relevant persons regarding a fire in your premises, you are not the RP for that part of the building. Your control ends at the ceiling, and that is where you can make the difference. A 60 minute ceiling is fully within your control, does not rely on any testing, and conforms to the standard set out in ADB. (Which does not list detection as a compensatory feature for a lack of compartmentation)
Cue arguments and claims of being unreasonable/jack booted etc........ (Go on, I can take it)