The SI highlights many things that individually would not cause an issue but in this case have all summed together in this incident. The captain needs to take responsibility for some and his admission of negligence goes someway. Although other crew members that day could possibly have prevented it.
There is also the 'cultural' side to it. How many flight crews do the correct hand over process, how many take unnecessary items on the flightdeck, how many spend excessive time away from the flightdeck, how many pilots left in sole charge don't become distracted etc.
In order to apportion blame, find culpability and give a fair punishment you need the full facts and evidence and also ask was his actions rouge and one off or was this a cultural problem across the RAF and the 'norm'
We are still not is possession of the full evidence that the CM has heard/seen.