Are we witnessing the decline of BBC Four's Friday night music documentaries?

It feels as if the strand is entering a predictable holding pattern reminiscent of the list shows Channel 4 used to throw up every Saturday.

Get a bunch of talking heads, make sure Rick Wakeman is included, clear the archive, and you're good to go. Recent programmes like Biggest Band Break Ups And Make Ups and Tales From The Tour Bus have exemplified the trend: they've been matey, but tend to repeatedly state their themes, rather than explore them.

Journalist Kate Mossman's film on pop fandom is another case in point. It's a likeable hour, charming in places, and throws up some good ideas. But it quickly loses focus, to blur into just another 60 minutes filling the slot.

Perhaps it fails partly because the ambition is big: there's too much history, hysteria and heated passion to get in.

Warning signs come early, with a narration that runs, "Never mind Sinatra and Elvis: the first truly global pop fan phenomenon erupted out of 1960s Liverpool."

It's difficult, hearing that, not to think what she means is, "Oh God. We've finished filming and somebody just pointed out I should have started with Sinatra and Elvis, but hopefully me saying this will be enough to get away with it."

Once onto the familiar ground of the Beatles, she gets going, with entertaining anecdotes of fans urinating on seats and the insanity of the Shea Stadium shows, by which point hearing the band was really not the point.

The Beatles reacted to that pandemonium by giving up playing live and retreating into the studio, and Mossman makes the brilliant point that it was here music fandom changed, or split in two, as, while many pop girls drifted away to scream at something else, "serious" fans became predominantly male, music-nerd types, obsessed as much with recording tech trivia as the songs.

From here, though, as the tribes proliferate, the narrative begins breaking down. There's sweet stuff from Alan Johnson about his years as a young mod.

But it seems bizarre to jump from there into prog rock as the next big moment: is it just so they can fit in comments from Wakeman?

It gets scrappy, where it could have been more interesting if it became a scrapbook.

Mossman frames the documentary with thoughts on her own mysterious girlhood obsession with Queen: some of the best moments are devoted to fan art she made in her bedroom and too-short readings from the loopy diary she kept, in which, in passages like Austen gone odd, lies recorded her overwhelming teenage crush on drummer Roger Taylor.

The best films about the nature of fandom just focus close like this on fans of certain artists - the goofy Judas Priest crowd in Heavy Metal Parking Lot, the Depeche Mode obsessives in Jeremy Deller's The Posters Came From The Walls, even Channel 4's Crazy About One Direction - yet in going so intensely specific, they can reveal something universal.

Not much is delivered in this film, but there are fragments to savour, with vivid sections devoted to Bay City Rollers headbangers, and contributions from greats like Siouxsie Sioux ("She was never a sex symbol," declares music writer Simon Price - I think I'll be the judge of that, thank you very much) and The Selecter's Pauline Black, recalling with amused horror the time a fan sent her pubic hairs in the post: ".... they were grey!"