Walter says ;
They were in control past the waypoint given for a turn by a navigation system; they could only reasonable have continued on if they had sufficient visual detail of the Mull to judge their distance off OR they were using another type of navaid for a closer in turning point.
1 We have no evidence that they were in control
2 No navaid would have allowed them to continue in IMC below SALT, so it is unlikely that they were IMC.
The point is, the evidence required to find negligence simply does not exist, in the same way that there is no evidence to prove the opposite.
No evidence = No Known Cause = cleared pilots.