I like this movie, especially Rod Taylor and the phony post-war world we get a few glimpses of, but the story suffers in parts from being needlessly overplotted to the point of undermining the whole premise.
Primarily I'm thinking of the audience suddenly learning, out of the blue, that Garner's and Saint's characters (Pike and Anna) are married. This is utterly pointless to the story; nothing is different than it would have been had they just become romantically involved. More to the point, the Germans were trying to make Pike give them the D-Day info; why distract him with extraneous aspects that have no bearing on their effort, which in fact could only interfere with it? By introducing such an unnecessary element the Germans were running a greater risk of something going wrong. As for the filmmakers, to spring this on the audience out of nowhere, and then basically just drop it as a plot line (almost no mention of it is made after the initial revelation, which is thrown in virtually as an afterthought), makes the film that much less focused and sensible.
Some of the interaction and competition between Gerber and Schack is also overdone, with needless plot threads tossed in to no purpose.
Unfortunately this was a tendency of writer-director George Seaton, who often overburdened his plots with pointless asides and developments that had no bearing on the story and just made the drama fuzzier and less believable.
And by the way, NO ONE with knowledge of the Normandy invasion was allowed out of Britain for months prior to D-Day. (And the German spy network of elderly people stalking Pike in London was laughably absurd.)