Walter, not for the first time you launch into a reply without ascertaining facts or reading source documents.
From the Release to Service, extant at time of crash;
Para 2.2 "GPS has not yet been declared operational by the US DoD and accuracy is therefore not guaranteed to any level. Even when GPS is declared operational by the US, accuracy of TNL8000 GPS (the system fitted to Mk2) could degrade substantially without any indications to the crew".
Para 2.3 (a) "In addition to para 2.2 above, the GPS is highly susceptible to jamming of which the only indication is loss of GPS..... The "ERR" figure displayed, which has conventionally been taken as a measure of GPS performance, is meaningless and so no indication of the accuracy of the GPS is available to the user".
There is no equivalent statement in the Mk1 RTS. The only reasonable assumption aircrew could make from this fact is that something had changed in the Mk2 that did not affect Mk1. As they were expected to operate a dual Mk1/Mk2 fleet, it was incumbent upon MoD to eliminate this uncertainty by finding out why there was a difference, agree what action to take and advise aircrew accordingly.
They didn't. They simply inserted a ludicrous statement that the "ERR" message was "meaningless". To me, that is a potential distraction, human factors risk, call it what you will. As I have said before, it must mean something, even if it is that the installation design is crap.
As you say;
there is clear need to focus on the basic nav just now
To me, it is a pretty basic failing if the system MoD's argument relies upon is "not guaranteed to any level". A logical extension of that argument is that Wratten and Days conclusions cannot be sustained to any level.