The side-issue of whether the story is true or not has as many twists as the plot itself. As mentioned above, all the families and the SAS denied they had been consulted or involved in any way with the book. However, an article the Daily Mail Online was forced to include an amendment when the widow of Major Mike Kealy admitted she had read and approved of every page that related to her husband.
The opening gun battle takes place in Mexico in 1980 yet the Mercedes that is attacked by the team (the one with the boy in it) clearly has 2010+ Mexican license plates.
Yes, it is based on The Feather Men by British adventurer Sir Ranulph Fiennes. The book tells the story of four British Army soldiers, including two members of the Special Air Service, who are assassinated by a hit squad known as "The Clinic". The murders are carried out over a 17-year period, on the orders of a Dubai sheikh whose son was killed by British forces in Oman during a battle with communist guerrillas.