Were any of the stories based on real life?
Yes, 'Hijack' where a consigment of East German silver is stolen was based on a real robbery in the early 80s. The character of Krivas in 'Where the Jungle Ends' is based on Costas Geirgiou, a notorious Greek-Cypriot mercenary from the Angolan civil war. The ' group in 'At Close Quarters' is clearly based on the German Baader Meinhoff terrorist group (also known as the 'Red Army Faction') and the assassins in 'Long Shot' and 'Mixed Doubles' are extremely similar to infamous international terrorist Illich Ramirez Sanchez aka Carlos the Jackal.
Why were the original actors playing Bodie and Doyle replaced?
Jon Finch was originally cast as Doyle but withdrew at the last moment as he did not wish to play an ex-police officer (ironically Finch was a former member of The Parachute Regiment and SAS, mirroring Bodie's background perfectly). He was replaced by Martin Shaw but when filming began the series' creator Brian Clemens felt that Anthony Andrews playing Bodie lacked chemistry with Shaw and that the two looked too similar. Andrews was dropped and replaced with Lewis Collins who had appeared together with Shaw in an episode of Clemen's previous series, The New Avengers.