Callan started out life as a one off TV play from 1967 entitled A Magnum For Schneider, from the pen of James Mitchell. It starred Edward Woodward as David Callan, a former soldier, miniature war games enthusiast and a dead shot, albeit reluctant, professional assassin for The Section, a branch of the Intelligence Service. ITV, quick to see they had a hit on their hands, ordered a series for later that year and the show ran for a total of four series until 1972. A film was made in 1974 and a one off TV special entitled Wet Job aired in 1981.
The theme tune, entitled 'Girl In The Dark', was written by Jack Trombey aka the Dutch composer Jan Stoeckart
It was subsequently covered by Chaquito and his orchestra and, with lyrics, by Edward Woodward himself - retitled 'This Man Alone'
The reverberations from ITV's successful spy drama would continue down the decades; Edward Woodward would go on to play a similar role in the US TV drama The Equalizer (Previously 'Theme Timed' here) whilst less savoury, the Cypriot Greek and former Corporal with the British Paras (alleged to have fired 26 shots on 'Bloody Sunday' in Derry, NI 1972) Costas Georgiou, was so enamoured with the series that he took the alias Callan (and the rank of Colonel) when he became a professional mercenary in Angola. He can be seen in the centre background in the photograph below. He was subsequently executed in the Luanda Trial of 1976 for killing 14 of his own men for 'desertion', 2 Angolan civilians and for various claims of torture.

PS; It's easy to forget how huge Edward Woodward was in his heyday. Callan was a national phenomenon. When one series ended on a cliffhanger, a graffiti campaign spread across the land to get the show back on air! He was the lead in cult favourite The Wicker Man as well as the lead in one of Australia's finest films, the anti war classic Breaker Morant. In the 80s with The Equalizer he was just as big, albeit internationally. He was the only white man to be allowed to walk unaccompanied in the downtown dangerous areas of New York's Harlem because of the good work his character did for minorities and he would always carry a card with details of helplines and support groups as he was inevitably asked for help by people confusing fact with fiction. The US, eager to keep a hold of him after The Equalizer wrapped in 1990, placed him in another thriller vehicle - albeit one in a very light hearted, naturally comedic vein - entitled Over My Dead Body, in which he starred as Maxwell Beckett a crime writer who is roped into solving the real thing by a young journalist played by Jessica Lundy. Despite it doing relatively well in France and here in the UK, it was a flop and cancelled after just three months. As a bonus, here's the theme/opening credits to that...
Ultimately Woodward returned to the UK and starred as a northern binman of all things in the excellent drama Common As Muck as well as a host of other great dramas and films (including Hot Fuzz) until his death aged 79 in 2009.