Continuity mistake: In the scenes in Bletchley (south of England) it is obviously early Spring (daffodils, few leaves on trees), but towards the end of the film, and apparently only a few days later, when a German U boat surfaces off the coast of Scotland, it is clearly high summer, with all the trees covered in leaves. Trees don't come into leaf that quickly, and the seasons are always later in Scotland.
Trivia: The twice-used shot of the train rounding a curve is actually near Rabbit Bridge on the Great Central steam railway near Loughborough. The train is in fact heading South; according to the film, it is first heading West (Cambridge to Bletchley), then North (Bletchley to Manchester). The same locomotive is hauling both trains - unlikely - and is suspiciously clean for wartime. However, it was a masterful touch to dub in a passing steam train as background noise when the security man interviewed Jericho in his digs. Such background noise is often conspicuously absent in films.