Gareth Jenkins (in-universe)

In the 1980s, Gareth Jenkins was an eight-year-old boy from Milton Keynes, Buckinghamshire, who wrote into the BBC One audience wish-fulfilment programme, Jim'll Fix It, to ask if they could help him to meet Colin Baker and see the TARDIS interior set in-person. As a part of the request, he demonstrated that he already had a complete Sixth Doctor's costume which his grandmother had made for him.

The producers of Jim took Jenkins' request further, and worked with John Nathan-Turner's staff at Doctor Who to create a sketch around the boy, A Fix with Sontarans. Jenkins — and his kid-sized Sixth Doctor's costume — did indeed play a major role in it, opposite Colin Baker and Janet Fielding. The mini-episode began – unlike most mini-episodes of the BBC Wales era – with series' full opening opening credit sequence. Janet Fielding's name was omitted from the intro entirely, as was the norm for actors on Doctor Who until 1996. In this instance, however, the intro displayed "starring Colin Baker", followed by "with Gareth Jenkins".

By the time the sketch was filmed, Jenkins' growth spurt in the interim was demonstrated by the prominent gap between his sweater-vest and trouser waist, and by his trouser cuffs exposing most of his yellow spats.

At the conclusion of the episode, Baker presented Jenkins with both a Jim'll Fix It medal and the "meson gun" prop to keep.

Jenkins' appearance is included among the archival footage shown in the 2006 documentary short, Built for War, an extra feature of The Sontaran Experiment DVD.

He is not related to the Gareth Jenkins who works with Big Finish Productions.

Gareth Jenkins, the "character"
Since A Fix with Sontarans so thoroughly breaks the fourth wall, it's difficult to consider Gareth Jenkins a character within the televised Doctor Who continuity.

Jenkins explains his skill with the TARDIS console came from having watched the Doctor operate the TARDIS on telly, but he does not mention the series, per se. (TV: A Fix with Sontarans) He could have been referring to a time-space visualiser which Barbara Wright called a "time television" [and, coincidently, watched Jimmy Savile on it]. (TV: The Chase part 1 "The Executioners") Thus, Jenkins could have conceivably watched anything the Doctor did during the Earth's relative past. (i.e. from the Big Bang circa 13.75 billion BCE (TV: Castrovalva) through the Fifth Doctor's 1984 parting with Tegan at Butler's Wharf. (TV: Resurrection of the Daleks'')

It is possible to give certain details about the Jenkins "character".
 * He is transported from Earth to the Doctor's TARDIS by accident.
 * He wears a boy-sized version of the Sixth Doctor's outfit.
 * He knows how to operate the TARDIS' controls from watching the Doctor do so on "television"
 * He kills the two Sontarans, because his viewings have told him which switches to press to kill the two Sontarans.
 * Group Marshall Nathan says that in the year 2001, when Sontarans invade, the adult Gareth leads a resistance movement which turns away the invasion fleet.