When Richard Hunter is in the pub another customer hands him a copy of The Eastbourne Chronicle - it's dated Wednesday September 14th 1940. Obviously this is a "mock-up" and whoever produced it didn't check their dates. September 14th 1940 was a Saturday.
A English character in this story is supposed to have been the inventor of the synchromesh gearbox. It was actually invented by an American, Earl Thompson, in 1918. He sold the patents to GM in 1924 for $1,000,000.
Paige departs on a military plane. His destination, as described earlier, would take him to Portugal and then on to the U.S. Portugal was neutral and, as such, would have confiscated any military plane that arrived and interned the pilot. Paige would have to depart on a commercial plane.
A man handling one of the stolen coins asks, "Who's Edward I-V?" Someone who doesn't understand Roman numbers probably would not be able to read the archaic lettering on a coin minted no later than 1483 -- and Edward IV's coins read EDWARDUS REX with no number.
The newspaper editor smokes filtered cigarettes. These were uncommon at the time and marketed chiefly to women. They would hardly be the smoke of a newspaper editor.
This episode is set in 1940, yet near the end, the transport plane shown is the C 47 Skytrain, known to the RAF as the Dakota. This is the most famous military transport plane of WWII. There were versions for paratroopers, infantry, cargo, and fancied up VIP carriers. The plane was not manufactured until 1941 and the 2000 Dakotas delivered to the UK for RAF use arrived in 1942. Furthermore, there is a mess plaque of the Parachute regiment on the wall. This regiment was not formed until 1941.
The photographer uses a Speed Graphic, an American camera. This would be improbable in itself, but the model (the "Pacemaker", easily identifiable by its white, rather than black, lens-board) was not marketed until 1947.
The date is September, 1940. Foyle goes into the local newspaper to get copies of photos taken at the site of a Nazi bombing. The editor says the photos have not been cleared yet by the government. As he is talking, and then after Foyle walks out, the editor is chain smoking, lighting one cigarette from another. Later, the American Howard Paige is also smoking. In all cases, they are filter cigarettes, which weren't in wide use until 1954. Throughout WWII, American soldiers were taught to "field strip" (shred) cigarettes so the enemy wouldn't know they were American cigarettes on the ground. They could do that with Lucky Strikes and Pall Malls, but filter tip butts are indestructible.
Colonel Letwin, who arrests the man suspected of being a German spy, is wearing his Sam Browne belt the wrong way. The strap should go through his right epaulette and not his left. It should cross his chest from top right to bottom left. It should not cover his decorations, as it does in this case. His holster should probably be on the other side as well but although this is not essential, it rather defeats the purpose of the belt which is to support a scabbard or heavy pistol.