The Guardian is a British national newspaper, published 6 days a week. Its companion newspaper, published on Sundays, is the Observer. The Guardian publish film reviews in their Saturday supplement The Guide, as well as online. They also publish a weekly podcast.
The Guardian is Mark's favourite newspaper. Indeed, whenever Mark airily refers to "a review I recently read...", Simon will often dig further to determine which publication was involved. 90% of the time it is the Guardian (see below for others). Similarly when Mark says things like "did you see that thing in the paper about...", The Guardian is always the paper in question.
The paper once managed to talk about "Frank Kermode's Film Review show" despite the fact that a) it is Mark not Frank, and b) it isn't his show. "Other than that, good reporting." Mark was particularly miffed as he already was writing a full-page column for the Guardian's sister paper the Observer.
The Guardian is famed for its spelling mistakes and errors, to the extent that Private Eye magazine refers to it as the Grauniad. Splendidly, a Wittertainment misprint during the Box Office Top 10 led to Simon asking Mark about a film called Rise Of The Guardian.
The Guardian crops up a couple of times in movies, leading to much mirth - especially its appearance in American Dreamz, in which reading proves to be eye-openingly revelatory for the Bush-esque President.