'Tinker Tailor Soldier Spy' is the first of what came to be known as the 'Karla Trilogy'. The second book was the 'The Honourable Schoolboy' and the third 'Smiley's People'. The BBC missed out making 'The Honourable Schoolboy' and went on to make 'Smiley's People'. It is assumed that the cost of filming the second in the series was the reason it was not made, as it is set almost completely in the Far East.
Michael Jayston, who plays MI6 officer Peter Guillam, played the title character in the British TV series Quiller (1975), about an MI6 operative.
Alec Guinness (Smiley) played Pol, Quiller's handler, in the 1966 film The Quiller Memorandum. George Segal played Quiller.
In John Le Carré's universe, MI6's headquarters are nicknamed "The Circus" after its fictional location in Cambridge Circus, a roundabout in central London that forms the junction of Charing Cross Road with Shaftesbury Avenue. The opening establishing shot of the series looks down on the real Cambridge Circus. (The roundabout has since been removed.)
Peter Guillam's car ("toy" in Smiley parlance) that he fetches Smiley to Lacon in is a Morgan Plus 8.
It becomes clear, as the series progresses, that Guinness's glasses are either clear glass, or very weak prescription. Distortion can be noted through the lenses, but only when the glasses are seen at an oblique angle to the camera.