"then there is no absolute proof that the aircraft was IMC and the case falls." I cannot speak for Brian and the rest, but I would be very surprised if they are hanging their hats on that. The primary objections surely are that the RAF's own recommendations preclude apportioning Gross Negligence to dead crews (and as far as I know that had not happened before or after, which is interesting) and the fact that there remains no proof of the same. If indeed the fog horn was not operating that would support the belief that at sea level the conditions were not a problem.