nigegilb:
SquidLord on a housekeeping point, only a few months ago the Nimrod IPT stated up front at a flight safety meeting that they regarded 00-56 as mandatory and it took priority over other source documents. Please state here if there are any great changes in the doc to which you refer. One thing that has been very obvious to me over the last few weeks is the perpetual state of change in the MoD and RAF.
If the Nimrod IPT were stating that it was mandatory for them, i.e. the IPT, to follow 00-56, then they do not understand MoD safety management (no surprises?) If they were stating that it is mandatory for, say, BAE to follow it then that makes sense (if it's called up contractually).
POSMS is the mandatory document for all MoD IPTs to follow in their safety management. It is much bigger than 00-56, containing much more detail, but follows the same principles broadly. Indeed, it inherits many of them, e.g. by using definitions filched from 00-56. This is actually arse-backwards. The MoD, or any organisation, should sort out its own Safety Management System (SMS) and then write documents concerned with the SMSs of contractors (00-56) accordingly, i.e. 00-56 should inherit the concepts of POSMS. But in the case of MoD, it's a historical accident - 00-56 has been around a lot longer than POSMS (the latter for just a few years).
Insofar as POSMS & 00-56 talk about the same concepts of safety management, there are technical differences but I think they are generally in the weeds and may even be unintentional. But POSMS contains much more detail and I would be concerned that any IPT that claimed that they were following 00-56, or that 00-56 takes priority might be missing important aspects of POSMS.