LINDA

LINDA (London Investigative ' N Detective Agency) was a secret organisation founded in early 2007, and broken up in the spring of that same year, due to all but one of its members being murdered. It was a society dedicated to finding out about the mysterious "Doctor", composed of five people: Elton Pope, Ursula Blake, Mr Skinner, Bridget Sinclair and Bliss, all under the guidance of the equally mysterious Victor Kennedy. The members of the group soon bonded and it became as much a social club as a Doctor-investigating club, much to Kennedy's annoyance.

All but Elton were absorbed by the Abzorbaloff (who Kennedy turned out to be), but Ursula was later partially returned from the dead by the Doctor's sonic screwdriver in the form of a face embedded in a slab of concrete. (DW: Love & Monsters)

The Doctor himself became aware of LINDA's activities before or during his fifth incarnation (at which time he appeared to find them slightly embarrassing). He briefly thought his tenth incarnation as a LINDA member, who he described as "a fan". (DW: Time Crash)

The Doctor made no attempt to contact LINDA until his tenth incarnation after Elton Pope befriended and then upset Jackie Tyler, which in turn angered Rose. (DW: Love & Monsters)

Other information
Other people tracking and commenting on the Doctor's activities include various fanzine writers (NA: Cat's Cradle: Warhead, Return of the Living Dad), Clive Finch (DW: Rose) and Larry Nightingale and his friends on the internet (DW: Blink).
 * Russell T. Davies came up with the acronym while writing another children's program, and re-used it in this episode.