Not sure the DC-3 story is completely correct. See this link:
https://core.ac.uk/download/pdf/2731742.pdf
On page 93 of 283 there is this:
"On 26 June 1941, Rear-Admiral Miles, the newly-appointed Head of the British Naval Mission to Russia, set off with a small naval staff by Catalina flying boat from Sullom Voe. Also aboard was the British Ambassador, Sir Stafford Cripps. In another flying boat was the head of the Air Mission, Air ViceMarshal A Conrad Collier, and the head of 30 Military Mission and his staff. "
"The appointment of Mason-MacFarlane as Head of 30 Mission was a blunder. The Foreign Office failed to anticipate adverse Soviet reaction to a lieutenant-general who, although he had distinguished himself in the Great War, was until shortly before the outbreak of the Second World War the British Military Attache to Berlin. The Russians took it for granted that the General was simply an intelligence officer...."
The flights were most likely to Archangel, Russia from Sullom Voe (Shetland Islands). Possibly a DC-3 flew them on from Archangel?