US Herk, my point is that it should not be necessary to have 2 ATC freqs on the go (the C2 and DZ freqs are clearly necessary for the task at the time). The 2 controllers might give conflicting instructions if they think they are both providing you with a service. Pick one and let the controller do the necessary liaison with other ATC units.
From the report, it says that Marham was providing a Basic Service (BS) to the Herc and passed traffic information on the GR4 on 2 other tracks. That implies that controller considered that there was a definite risk of collision (rules for a BS) but then he stated that the severity of the incident was negligable, which does not add up. The Herc was also receiving a TS from London Mil; why is there no input from that controller in the report?
As for their NOTAM, as published it only told folks what was going on, it did not say 'keep out'. Some sort of airspace reservation would be better. I would say that a permanent avoid at Sculthorpe activated by NOTAM would be the way ahead.
I speak as an ex London Mil East ATCO who gave regular UK ATC/airspace briefs at Mildenhall and Lakenheath, has been on a LZ/DZ party many times and has been meat-bombed and TALO'd into Sculthorpe on a few occasions.