[2] [need quotation to verify] From Richard Tomlinson's book The Big Breach, detailing the accounts of his experience as an SIS Intelligence Officer, the basic facts about the composition of the Increment and its relation to SIS are noted: The army provides a detachment from the SAS regiment, called Revolutionary Warfare Wing in Hereford, and the navy provides a small detachment from their Special Boat Service in Poole.
To qualify for the increment, SAS and SBS personnel must have served for at least five years and have reached the rank of sergeant.
Advanced civilian qualifications are acquired: several of the SBS Increment have commercial ship's skipper’s tickets in their alias name, enabling them legally to hire, say, a fishing trawler.
[8] From the personal blog[3] of former British Army 22 SAS Soldier Colin Armstrong best known by his pseudonym Chris Ryan: I've been asked if the Increment is real, and if the kind of deniable 'black' ops I depict in my novel of that title really happen.
They have to undergo incredibly stringent background security checks because they are entrusted with the most sensitive operations within the entire military.