After checking the regulations I think I was wrong to about a smoke alarm outside the flat. The regs require the landlord to "ensure ... when the premises are occupied under the tenancy...a smoke alarm is equipped on each storey of the premises on which there is a room used wholly or partly as living accommodation". I think the "storey of the premises" is more likely than not to be read to mean "in the premises on each story". I could be wrong. But a quick search found guidance one way or t'other.
TLDR: I think you need a smoke alarm in the flat unless your LA is happy for you not to provide one.