> The Registry employed enormous numbers of girls to maintain efficient delivery of files within the building, as well as the massive task of sorting, checking, and filing the incoming material. In Kell's day the Registry Queens, as they were known, were recruited either from the aristocracy or from the families of MI5 officers. Kell had a simple belief that this was the best vetting of all. [...]
> By the early 1970s the staffing of the Registry had become a major problem for MI5. There were more than three hundred girls employed and with the surge of file collection at that time the pressure for more recruits was never-ending. Openly advertising was considered impossible. Yet it was becoming very difficult to recruit this number of girls, let alone vet them properly. In at least one case, the Communist Party managed to infiltrate a girl into the Registry, but she was soon discovered and quietly sacked. This problem, rather than dissatisfaction with the increasingly antiquated filing system itself, finally pushed MI5, belatedly, into accepting a computerized [sic] Registry.
As the quote hints, large landholdings and a family with long standing ties to the country (i.e. old money) are great ways to ensure loyalty, both emotional and financial, and loyalty is much more precious than ability in intelligence work.
The Cambridge Five were a very public (and rare, if costly) failure of this system, and Yuri Modin (their handler) claims  that the tension between the middle class, Scottish-accented John Cairncross and his upper class colleagues was part of what pushed him over.
The British merchant banks' eventual deaths at the hands of meritocratic American ones is strong evidence that this is not a good system in areas where ability is more valuable than trust.
 Peter Wright, Spycatcher - https://www.amazon.co.uk/Spycatcher-Candid-Autobiography-Int...
 Yuri Modin, My Five Cambridge Friends - https://www.amazon.co.uk/Five-Cambridge-Friends-Cairncross-C... - Modin died a hero in Moscow after a prestigious career including heading the KGB's Active Measures department, so it could be mostly disinformation, but his observations on human character are still great.
- The Man Who Mistook His Wife for a Hat ~ Oliver Sacks, 1985. This book contains tales of some of the Sacks's patients. A very interesting read. 
- The Mind's Eye ~ Oliver Sacks, 2010. 
- Spy Catcher (Autobiography of a MI5 agent) ~ Peter Wright, 1987. 
- Applied Cryptography ~ Bruce Schneier, 1994. Approachable and succinate language of this book makes it easier to understand. 
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