Safety Cases came in, possibly in the late 90s
This touches on a much misunderstood aspect. While Safety Cases were mandated in the early 1990s, that is not to say safety was ignored before that. It was just called something different - the Safety Argument for example. I still have my copy of the mandated regulations procurers were under, dated 1977. They underwent a major update in 1985, which added chapters; but the original 10 remained extant and mandated. (For example, managing software projects became a bigger aspect of our work and specific regs were added). In the section covering contract acceptance, safety features heavily. In other words, and regardless of what the bit of paper was called, if the contractor couldn't prove the product met its safety criteria (set out in the project plan from day 1) then it was not accepted off contract by procurers, and (obviously) could not be offered to the Service and accepted into Service. Nor, obviously, could a Release to Service be issued (legally).
All contractors understand this, and very few in my experience would remotely consider defrauding MoD by falsifying safety reports. (I'd like to say "none", but unfortunately I know of two - both led to multiple deaths and injuries, but no action was taken). The problem, admitted by MoD in the Service Inquiry report, is that this process broke down catastrophically in the case of the Mk10 ejection seat. I'd bet my house Martin Baker has a valid Safety Case. As I'd still have the house, I'd bet it again that MoD has not had Martin Baker under appropriate contract, so the MoD Safety Cases have lapsed or just been ignored - and this is what the SI refers to when it admits there is no Safety Case.
In other words, precisely the same as happened on Nimrod MR2, when MoD didn't bother contracting its upkeep for many years, and when an attempt was eventually made to resurrect it, it employed a safety manager who hadn't a clue, as it had been so long since MoD trained anyone. (And therein lies the reason he, and his RAF officer colleagues, were not prosecuted). Which gets us back to XX177 and why (a) the Crown prosecution Service determined that no case could be brought, and (b) the Health and Safety Executive have decided to pursue Martin Baker but not MoD, despite the latter openly admitting serious offences. Something's going on.