To answer your question Kurnal the Fire Safety Manager should have a broad knowledge of fire safety matters, but will often rely on the informed guidance from an advisor on the nitty gritty detail.
The exact role of the Fire Safety Manager is clearly defined in the HTMs - I wont write the definition here as it will take up too much space but I can happily email it to you if you wish. It defines the minimum attributes / knowledge expected of the FSM
In short it says that the Fire Safety Manager should take the lead - it is in some respects a figurehead role - it exists to give some clout to get things done when needed.
Often the fire safety manager will actually do another role in the Trust. One Fire Safety Manager I deal with is actually the Trust Estates Manager for example.
On to your second point about fast tracked personnel or those recruited without the necessary underpinning knowledge because it is related to this discussion.
You know yourself Kurnal that on promotion Brigade Officers can be put in charge of departments they have little experience of such as Fire Safety, Fire Investigation, Brigade Communications, Workshops, I.T. etc etc, The fact is they exist primarily to manage - sometimes the role will need a wealth of experience in that field (An Incident Commander requires operational experience to do their job. But does an Officer need operational experience to manage a fire safety department? ) Sometimes it will call for informed decision making based on guidance and advice from colleagues instead of indepth underpinning knowledge.
Take it a step further - lets say your in charge of a HAZMATS incident - you come across a chemical you have never heard of before - so you request a Hazmats officer to come on scene and advise you of what to do. It doesn't mean to say you r incompetent, you just need additional guidance to help you make informed decisions.
I liken that to the relationship between the FSM and the Fire Safety Adviser.