In the two-film archival section of this year's Sundance Film Festival, one British director shares the limelight with an American director, Gregg Araki, whose leftfield and largely gay-themed films would probably never have existed without him. His name is Derek Jarman, whose "Edward II" (1991) is being screened in the Sundance Collection slot, and he is also the subject of one of the two UK entries in the World Documentary Competition. Simply entitled "Derek", it provides an affectionate tribute to a director who pioneered the arthouse scene of the late 1970s and early 1980s, providing a rough, rude and rebellious counterpoint to the sleek, mannered and conservative movies – the Merchant-Ivory travelogues, and the Chariots Of Fires – that came to typify the British film industry at that time.

Admirers of this cultured, warm and funny man will find plenty to enjoy in Isaac Julien's film, which rests squarely on a long and candid interview conducted by Colin McCabe, shortly before Jarman's death from an Aids-related ilness in 1994. In it, Jarman talks frankly about his youth, his move into the art world and his accidental break into cinema after working on "The Devils" as a production designer for Ken Russell. Such memoirs are a breath of fresh air in today's PR-controlled climate, and no subject is off-limits, from his parents to the tricky subject of his sexuality, which Jarman admits was not something he allowed himself to address until the age of 22.

This is all well and good, but there are two niggling problems with this accomplished, MOMA-commissioned feature. The first is the presence of actress Tilda Swinton, once Jarman's muse, and though her posthumous letter to Jarman, read as a voiceover, certainly conveys her fondness for the man, it hints at a rich seam of information that the film never fully explores. Together, they were a dynamic team, as can briefly be seen in Jarman's dream-like Super-8 home-footage, and it would be nice to know more about this clearly passionate collaboration; an unlikely double act who had discussions about art, history and ancient civilisation while calling each other "Del" and "Tild".

The other problem is more one of tone, and though it may pass as classic, Julien's film could just as easily be labelled old fashioned. In the 14 years since his death, Jarman's work hasn't really been reappraised, and this is not quite the film to make that happen. "Derek", as a film, rests squarely on the assumptions that surrounded Jarman in his enfant-terrible prime: that his films were fresh, fabulous and terribly, terribly important. But, looking back, were they? Making his swansong, "Blue", just as his health was about to fail him for the last time, Jarman quietly announced that he was switching to painting, and his artwork is briefly glimpsed in the closing moments, as Jarman potters in his Dungeness getaway, perhaps the most accessible art statement he made in his life.

For this reason, though it functions perfectly as a fond farewell from friends and peers, "Derek" falls short as a film in its own right simply because it never really questions its subject's achievements and reputation. More crucially, it's not likely to reinvigorate Jarman's legacy for a new generation, who take gay politics for granted and might take more from his paintings and elegant notebooks than they would from his films, an acquired taste even in his heyday.

Editor's choice

• Vote for the Breakthrough Award winner
• David Tennant back as Hamlet after back surgery
• Matt Smith is the new Dr Who