How successful was the Security Service during the early part of the twentieth century?

When addressing a question such as this, it is imperative that its terms are defined. In this case it is important to explain what is meant by the Security Service, and also what is meant by the early part of the twentieth century. In October 1909, following a recommendation by the Committee of Imperial Defence which had been considering the danger to British naval ports from German espionage, Captain Vernon Kell and Captain Mansfield Cumming established the Secret Service Bureau that became MI5 in 1916, along with the addition of many other specific sections devoted to the intelligence service in Britain. Naturally then, October 1909 should be the starting point of the period to be covered, and the end point, the outbreak of the Second World War . We must also be clear about what is meant by success. The primary role of the Security Service was to protect Britain by preventi…(short extract)