What almost everyone ignores is the fact that negligence was found by OC RAF Odiham in his capacity as one of the first Reviewing Officers.
He found that Flt Lt Tapper (as Aircraft Captain) "had a duty of care" in respect of his Passengers and Crew and that Flt Lt Tapper had failed in that duty.
Chambers Dictionary - Negligence
"Omission of duty, especially such care for the interests of others as the law may require"
AOC 1Gp concurred and extended that finding to Flt Lt Cook.