The dating of the film is imprecise: The hotel register implies an August setting (with a newspaper confirming the South of England is in the grip of the hottest August on record). The film opens on Monday, but on Wednesday Mrs Peckett's newly-delivered "Daily Express" is dated Saturday August 9th. The following day, Goldie's "Daily Graphic" is dated Friday August 11th or 14th, whilst the Downing Street calendar more accurately shows Thursday the 12th. It can be inferred that the film is meant to cover Monday 9th - Sunday 15th August 1950.
The Special Branch dossier on Willingdon gives his forename as John Francis, with an 1893 birth date (making him 57, as the film explicitly occurs in 1950), but his Wallingford security ID names him John Malcolm, aged 55; later David refers to him as Professor JT Willingdon.
The film is said to take place in 1950. However, some of the hotel register entries appear to be dated 1952.
The police notice about Willingdon is 100% accurate as to his height, complexion, eye colour etc., yet can only provide "Age about 60". It also notes he has a scar on his left shin, whilst his security details note "Nil identification marks".
On Monday, Folland's door reads SPECIAL BRANCH C.I.D. with his nameplate (and he has no office clock); by Wednesday this has changed to SUPERINTENDENT. The clock is present, though seemingly stopped at ten to two.
Further to the comment of "In 1950, a nuclear weapon would weigh at least six metric tonnes. It certainly would not fit into a small Gladstone bag as shown in this movie" - the word "nuclear" is never used in this film. It is clearly stated that it is a new secret weapon, a UR 12, which does fit into a small Gladstone bag. It isn't an atom bomb (the term used at that time).
In 1950, a nuclear weapon would weigh at least six metric tonnes. It certainly would not fit into a small Gladstone bag as shown in this movie.
As Folland dispatches Lane to look through the papers at Green Gables, the reflection of the crew/onlookers can be glimpsed in the police car windows.
In the opening scene, a postman (despite having a large bundle of envelopes on location) delivers six letters to 10 Downing Street. However, there are seven on the floor behind the door, as the production team have ensured a pre-positioned one is face-up and easily legible.
The prop hotel register for the Imperial Hotel, which is held directly in front of the camera, is misspelled, and says "The Imerial Hotel."
Goldie hails a cab and asks the driver to take her and Professor Willingdon to her home in Kennington. When they get there Willingdon can be seen looking out of the window towards St Pauls Cathedral, which in real life is around 2 miles away, and not visible from any location in Kennington.
At the end of the film, Goldie finds herself on Westminster Bridge walking east (ie towards the south side of the River Thames), still trying to get a car to Aldershot. When the all clear sounds and she realises she can go home, she turns round and heads west, in the opposite direction to her home in Kennington.