He should never have gone IFR. He knew before he took off that the aircraft was not cleared for it.If the weather was as good as the yachtie said, he could see the fog shrouded Mull and should have slowed down, turned away, reduced height and so on. If he was able to
Syllogism
Any competent aircrew would have turned away as described, IF THEY WERE ABLE TO.
Tapper and Cook (and the other crew) were competent, in the view of all who knew them.
Therfore they were unable (for some unknown reason) to carry out the expected action.
Thank you Baston for another nail in the coffin of the totally unjustifiable verdict of Wratten and Day.