There are 11 episodes, from the first two series, for which there are no known copies in existence, although full unedited studio recordings do exist for The Worst Soldier I Ever Saw (1969) and an off-air soundtrack for Once a Big Man, Always a Big Man (1969) was recovered in 2019.
The swinging lightbulb in the opening titles was not merely some attractive credit sequence design: The Section regularly made use of a 500 watt lightbulb swinging in the face of interrogation suspects to help disorientate and frighten them.
Russell Hunter played London black-cab driver Lonely, but in real life Hunter could not drive and did not have a driving licence. All the shots of Lonely driving the taxi were filmed with it on the back of a low-loader that was towed around the streets of London.
To maintain the harsh and grim style of the series, nearly every episode was shot during the winter months in order to achieve this effect.
The series theme tune was 'Girl in the Dark', a stock library piece by Dutch composer Jan Stoeckart (as Jack Trombey), published by De Wolfe Music. In 1970 Edward Woodward released a vocal cover of the piece, 'This Man Alone', with lyrics by Peter Dacre. The track was subject to a copyright infringement claim from Mood Music in 1967, who asserted it copied their piece "Sogno nostalgico" by Armando Sciascia; De Wolfe eventually lost and paid all costs, estimated at £70,000. This other work is still sometimes wrongly identified as the series' theme.