The Nimrod Safety Case (NSC), which is not required for legacy systems before the HSAW Act, was carried out because the MOD's JSP indicated that, in general, it would be sensible to gather as much data as possible in order to provide an argument that the legacy aircraft also met the principles required under the HSAW Act.
EdSett, you have produced some of the more informative posts on this thread, but this is Twaddle !
Let's get something straight, THERE IS NO LEGAL REQUIREMENT FOR A SAFETY CASE. However, many regulated industries require a Safety Case to be produced and submitted for regulatory approval. The MoD has chosen this approach and has been using it for some time for new equipments, it has been well documented in successive issues of Def Stan 00-56. For legacy in-service aircraft, the MoD mandated the production of a Safety Case in (then ES(Air)) Business Procedure 1201.
S_H