Walter Kennedy,
d246 has brought the focus of the main debate back to the real issue and I hope you will see how your interesting speculation about the use of navaids is not relevant and may be seen by some as distracting from that issue.**
Wratten has clearly justified his decision by claiming that ZD576 was travelling too fast for the conditions at waypoint change. This view has been clearly supported by several contributors including john purdey and, now it appears, d246 and cazatou. That is the sole issue upon which he bases his argument (Pilot article written by him)
The only evidence we have about that point in time is that, shortly before, they were seen by Mr Holbrook to be clear of cloud and in sunlight. He says that he could see the base of the lighthouse two miles away. We also know that the lighthouse keeper was in cloud. As you say, it is likely that this was orographic cloud and was extremely local to the upward slopes of the Mull.
There is also clear evidence that ZD576 had experienced various technical problems which had not been solved at that time and anyone of these could have, at the least, led to serious distraction.
Everything else is speculation!
Many times, john purdey et al have been asked to explain how they know, beyond any doubt, what the exact conditions were at waypoint change, as seen from the cockpit by three very experienced aircrew. So far, they have refused to answer that question. Perhaps d246 will do so now.
Until they do, I remain in considerable doubt as to the safety of the Senior Officers judgement.
** wk, if you are suggesting that the crew of ZD576 deliberately flew 25 passengers into IMC below MSA with two conflicting navaids, you are indeed suggesting gross negligence, in my view.