The consensus was that the meeting of the two leaders could be kept secret for several days, but not for the whole ten days the Conference was due to last, and a cover plan was devised whereby about half way through the Conference news would be leaked out that it was taking place, but in Khartoum.
The first task however was to keep as many people as possible in ignorance that not only Churchill, but large numbers of his entourage had disappeared as well.
It transpired that some 700 people had access to the minutes of the Chiefs of Staff's daily meetings.
DW suggested that the minutes be fabricated, and it was agreed that when - for example - the Vice Chief of the Imperial General Staff spoke in his Chief's place that it would be minuted that his Chief, Sir Alan Brooke, had spoken instead.
Various potential leaks had to be plugged - DW had the task, for example, of breaking the news to Churchill's doctor that he must cancel an order he had put in to his tailor for two tropical suits.
As it happened though, the biggest potential leak was made by the Chiefs of Staff themselves. Having boarded the train that was to take them to the airport in great secrecy, when they were dropped off in Berkshire for a fleet of cars to take them to the airfield, they found they had three quarters of an hour to spare. Alan Brooke, Portal, Mountbatten and a score of others prompty made off to the nearest pub. MI5 had to quarantine it for a month.
For DW's account, click here
It was however an interesting world of secrets, and people did not always know 'who knew what'. An example of this concerned the tightly held knowledge of 'Ultra', or that the British had broken various top secret German codes such as Enigma. It was once disputed whether DW knew about the fact that the codes had been broken, or whether he was just fed the results, which would have been necessary for his work. The matter was recently resolved (September 2020) during an analysis of DW's typescript of the book we now know as 'The Deception Planners', which compared this to the (posthumously edited and) published text. It appears DW did know about Ultra - but not from Johnny Bevan. To read DW's account that did not make it into the published version, click here.
References : The Deception Planners pp 112-116
Phil Baker pp 245,297-303,330,460.
Craig Cabell Chapter 24.