How can you tell if a risk assessment is suitable and sufficient if you do not inspect the whole building in order to ascertain if all significant risks have been identified and assessed?
You can`t entirely, I assume your brigade uses the CFOA guidance on the audit process. This document covers the audit principles, how to carryout the audit and the verification of standards.
Verification of standards must include a risk critical constituent, then if you find major deficiences in this area then a full inspection of all risk critical constituents is to be made. You document the area choosen on the audit form to cover your brigade (oh sorry I meant yourself) the FSO has an advantage here over the consultant he should have years of history in a file.
Then you use the nationally agreed 'get out letter' that states that:-
I am pleased to advise you that, at the time of the visit, a satisfactory standard of fire safety was evident. The visit was not a comprehensive audit of all fire safety matters but looked into a variety of aspects from which our findings are drawn.
You should be aware that you have an ongoing responsibility to maintain that standard and to carry out periodic reviews to ensure that safety provision remains adequate.