Crowley: You had quite a reputation in your day, Flint. Don't ask me why, but you all did. That's part of why you've been returned to active duty in the Paranormal Investigations Squad.
Flint: The PIS, sir, yes.
-Entropy in the U.K.

Division X is an organization in Grant Morrison's comic book The Invisibles.

In the late 1960s, three members of the Invisibles form a squad under the British Paranormal Investigation Service(PIS). Upon their decision to do this, they decide to go under deep cover and take upon personalities based on characters from TV shows. The squad was composed of Jack Flint, George Harper, Mister Six, and was supervised by Sir Patrick "Paddy" Crowley.

"Christ, but you're an undeducated bastard! Flann O'Brien only wrote "The Third Policeman," which is one of the greatest books in the English language. You'd like it, George, it's a comedy, like your bloody haircut."
-Jack Flint, Entropy in the U.K.

Jack Flint based his new personality off of Jack Regan from The Sweeney. He had some minor psychic powers, however, his strong points were his ability to commune with spirits as well as not having any qualms with interrogating suspects in a rough manner.

"I used to envy those bastards up in vice; getting to watch this stuff all the time. Dirty buggers!"
-George Harper, Entropy in the U.K.

George Harper, in actuality an anarchist named Eric Millet, based his personality on George Carter from The Sweeney. The fact that both Harper and Flint's based personalities existed in the same show might be why they feel so close to each other. Harper is a calm, happy man and enjoys the thrill of being in Division X. He has low-level general psychic abilities, but has a very strong ability in precognition and psychometry.

"Police. Stay perfectly still while I read your minds."
-Mister Six, The Invisible Kingdom

Mister Six, also known as Jon Six, takes on the personality of Jason King in the series Department S and Jason King. Six lives his life like a playboy, and generally looks like he's still living in the 70s. The complete Mister Six has powers greatly beyond any of the other members, but in his Jon Six personality, he tends to only show a high level of telepathy and perhaps some precognition.

Division X worked up until 1976 when an enemy discovers the identities of the members. Whether this means the true identies or not is not revealed, however, one can probably assume not since Jack and George are still in their Division X personalities when it's reactivated. The final result is that the group is disbanded.

Crowley: Something happened here, in this shop. We're not sure what, but of the three sensitives we brough in, one is now wearing a straitjacket, the second has declared himself the messiah... and the third remembered Division X.
Flint: Must have been a fan club member.
-Entropy in the U.K.

In 1995 at The House of Fun, a psychic recalls Division X after attempting to read the place. Paddy Crowley is called in to reinstate the group. Flint is the first one notified(Apocalipstick), who then calls in on Harper(Entropy in the U.K.). Six is the last one to show up for their first new meeting.

Division X's investigation in The House of Fun leads them to pornographic videos of "birds having it off with aliens."(Entropy in the U.K.) The person that made the video is a man named Quimper, who apparently has ties high up so far as Sir Miles Delacourt. For the next couple of years, Division X is tracking the movements of Quimper and Sir Miles in order to find the alien in the pornographic videos.

In 2000, Jack Flint faces a problem. The Jack Flint persona is to die of a heart attack. In order to save Flint's life, Mister Six abducts him and tries to make him remember his true identity as an Invisible. It also comes out that Harper actually did remember his own identity, and was rather pissed at Mister Six for abducting Flint.

In order to finish their case, all of Division X assists the other Invisibles in the final assault at Westminster Abbey(The Invisible Kingdom). Flint dies in the conflict, but Mister Six and George Harper give a toast to their absent friend and to the new adventures of Division X.

"Bloody hell, guv'nor. This used to be easy. Remember we just used to get in the car, sort out some unconvincing ghost and then get pissed and chase birds all night?"
-George Harper, Entropy in the U.K.

Sources:
Anarchy For The Masses, Patrick Neighly and Kereth Cowe-Spigai, Mad Yak Press.