Having now read the HoL report I am inclined to agree with AirfixPilot that pressonitis was a factor. The cloud base crossing the North Channel must have been lower than they were expecting. The report mentions a visibility of less than 50 metres at the lighthouse. This means the cloud base must have been less than 400 FT. If the Chinook was at around the same height it would have been very difficult to see anything.
The statement from the yachtsman eye witness must be taken with a pinch of salt - he saw the Chinook and the Mull so they must have seen it as well. What about the old chestnut of airbourne flight visibility.
Given the poorer weather they encountered and the problems of climbing into potential icing conditions, turning back might have been a better choice but they had the passnegers!!!!
At the end of the day though all this is supposition and with the lack of hard evidence I also find it incredible that to BoI came to such a dogmatic conclusion.