Weather on Mull
This discussion comes down, as it has always done, to what is 100% proof. We are splitting hairs extremely finely because it is only a split hair between 99.5% probability and 100% certainty.
BOAC
It seems to me that SFFP is arguing that because there was no-one who saw the actual crash and that it is conceivably possible that the aircraft was VMC, then there is no absolute proof that the aircraft was IMC and the case falls. With respect, I do think that the weather is one of the issues.
Tandemrotor
If one walker had reported being in fog, he could possibly have been in a 'fog patch'. If ten walkers on the hill in different locations all stated that they were in fog then they were not standing in a 'fog patch' but on a fog-covered hill. For the aircraft to have been operating within limits there would have to have been a corridor giving 1000 metres viz in all directions with a 250' cloudbase that was not evident to any of the walkers, never mind the guy 100 metres away.