SKUL

SKUL (Secret Kriminal Underworld League) was a self-declared criminal organization that appeared in S03E02, "Spy For A Day". It was led by The Grand Master, who was revealed to be Mr. Flatley in disguise. It was disbanded after Avril Franklin murdered the Grand Mistress.
It claimed to have formed in 1969 and been in operation for the entire time, although Avril found this extremely suspicious. 
When pressed, the "Grand Master" then claimed that The Beatles were early partners with them and that 'Maxwell's Silver Hammer' was secretly about a SKUL agent going around taking out M.I.9 spies one by one, and Ringo Starr invested his marriage with Maureen Cox into the business.