From TNA, a synopsis with references to the original files, on Lachmann from Security Service files released in 2008.
Gustav Lachmann (KV 2/2733-2735)
Lachmann, who was a German air lieutenant in the First World War, became an outstanding aeronautical engineer during the inter-war years. He worked in Germany and then Japan, before marrying his British wife in Tokyo and settling in Britain, where he was engaged on aircraft design by Sir Frederick Handley Page. This reconstituted and heavily weeded file records the deep suspicion with which Lachmann was viewed, as a potential spy for both Germany and Japan.
KV 2/2733 (1928-1936) shows how Lachmann's mail was intercepted to look for signs of espionage activity – none were found, but efforts were still made to control his activities with Handley Page Ltd. By 1936 (KV 2/2734, 1936-1942) it was noted (serial 197) that "It appears undesirable that a German should be employed in the works of a firm carrying out contracts for the Air Ministry, but…enquiries for over two years have produced nothing to lead to suspecting that he is using his position…to give information either to Germany or to any other power." Sir Frederick is named as employing "verbal cunning" to defend his continued employment of Lachmann, but the pressure on the firm grew until in November 1938 the Secretary of State instructed Handley Page to dismiss Lachmann (serial 358x).
Lachmann was interned at the outbreak of the Second World War, though while he was on the Isle of Man arrangements were made for him to continue working on aircraft design (e.g. serial 595a in KV 2/2735, 1942-1955).
By 1944 consideration was given to Lachmann's post-war employment, with the fear that he might go back to Germany or to the United States. Eventually he resumed his work with Handley Page.
He was clearly viewed with deep suspicion by our Security Services in the period before the war, though these suspicions were evidently unfounded.
I took this photo in 2018 when I was over that way. I got a good look around it and was able to poke my head inside. It looks great and on the ground like that you get to really appreciate just how snug the crew compartments are in the Hampden. It's progressed a lot more since that photo was taken though.