TD, getting the BAES safety case may not provide the answers you are after. JSP 553 has 3 level of safety case:
1) that produced by the designer (essentially covering what they have sold you)
2) that produced by the IPT - The IPTL's safety case consists of the Designer’s safety case for the as-built standard of aircraft plus the safety justifications used by the IPTL to underpin his certification of the initial draft RTS or, for legacy platforms, the issue of the MA Release.
3) that produced by the Release to Service Authority (RTSA)- The RTSA’s safety case will initially comprise the IPTL’s safety case plus the safety justifications used by the RTSA to underpin the issue of the RTS and is applicable to the configuration of the aircraft ‘as-flown’ by the Service.
tuc, there is nothing wrong with the concept of the top down approach as long as it goes far enough in getting the evidence required. I understand that some well known people in the field were brought in by the IPT to provide a top down argument in Goal Structuring Notation. However, I know nothing of the content of that argument.
sw