Others have discussed the plot and acting in "Cottage to Let" (aka, "Bombsight Stolen"). To tell too much would take away from the enjoyment of this film. Some have said it has a slow start. But without such a background and build-up for so many characters, I think we'd be lost. At least one reviewer doubted the probability of such a scenario. I agree with the majority that this is an excellent war mystery and spy thriller. The cast is superb, with some big names of English theater and filmdom – John Mills, Alastair Sim, Michael Wilding, Leslie Banks, and others. And, it has an excellent supporting cast.
Of course, this is a fiction story, as are so many of war-time. But as to the likelihood of something like it happening or not, one should consider some other factors. This movie was released in England on Sept. 6, 1941. The U.S. was not as yet in the war, even though most of Europe by then had been overrun by Nazi Germany. The official start of World War II was two years earlier. On Sept. 3, 1939, Britain and France had declared war on Germany after it invaded Poland.
The Battle of Britain was waged from July 10 to Oct. 31, 1940, with Germany bombing London, major ports, and other large cities. Even after Britain won this battle for air superiority, Germany continued to bomb London and other cities. As this film noted, Londoners sent their children to country locations to keep them safe from the bombing raids. And, in fact, many British secret operations, including research and war design work were in locales across the country – away from the population and large military bases.
Even after the U.S. entered the war and began sending troops to England in 1942, the Allies continued to disperse many of their war-time operations across the countryside. Many special projects were going on, none of which would be common knowledge to the public or reported in the press at the time. Only after the war did we learn about them. Movies have been made about some even decades later. All are interesting tales. Among the ones I've seen and enjoyed are: "Secret Flight" (aka, "School for Secrets") in 1946; "The Small Back Room" (aka, "Hour of Glory") in 1949; "The Dam Busters" in 1955; and "Enigma" in 2001.
England had its share of German spies. British intelligence agencies broke up some German espionage rings working for the Abwehr, the German military intelligence service. And, Germany had tried to land agents by submarine in Scotland. No one knew or could imagine to what extent German agents or spies may be operating in England. So, this film was timely as well.
I am curious though, about the late date of release of the film in the U.S. – May of 1943, Most of the British-made films during the war were released a year or more later in the U.S. One reviewer said that the Brits preferred American war films to those of the British film studios. I doubt there is any movie attendance or other data that would lend credence to such a statement. I'm sure the British public was drawn to all the war films that were being made at the time, regardless of the country source. No doubt, Englanders wanted to see some of the American movie stars they had come to know. At the same time, British studios were putting out some excellent films. Among them were "One of Our Aircraft is Missing," "Went the Day Well?," "In Which We Serve," "The Way Ahead," "49th Parallel," "Fires Were Started," "Convoy," "Freedom Radio," "The Day Will Dawn," "The Next of Kin," "The Foreman Went to France," "The Bells Go Down," "The Silver Fleet," and "Undercover."
Many of the British post-war films also were excellent. I enjoy these films immensely, because they give us a look at the war from the eyes of British servicemen and public. Just as American films give others a view through Americans' eyes. The quality of the DVD I have with this film is rather poor. I hope a digitally mastered DVD will be produced one day soon.