The real Ewen Montagu had a "surreal" cameo role in the film
The screenplay of the film stayed as close to the truth as was convenient, with the remainder being fiction. For example, the Irish spy in the film is a complete fabrication. Ewen Montagu declared that he was happy with the fictitious incidents which, although they did not happen, might have happened. During filming, Montagu has a cameo role, that of a Royal Air Force air vice-marshal who has doubts about the feasibility of the proposed plan. It was described as a "surreal" moment when the real Montagu addressed his fictional persona, played by Webb.
The difference between stupidity and genius is that genius has its limits. -- A. Einstein