A Mighty Wind is very much business as usual for director Christopher Guest and his acting troupe. Picking up whereThis is Spinal Tap left off fifteen years earlier, this is the third offering in a cycle of mockumentary comedies from Guest and co. (beginning with the never-released-in-the-UK Waiting For Guffman and continuing with Best In Show), and the formula is beginning to show.

Once again, Wind revolves around the build-up to a make-or-break public performance: in this case, three 60s folk music acts brought together for a reunion concert following the death of legendary folk promoter Irving Steinbloom. And, once again, the subjects in question are gently satirised as they inadvertently reveal their quirks to an intrusive documentary crew we never see. The three acts, as with Best In Show’s dog-owners, have contrasting approaches and attitudes to their craft; the slick professionalism of The New Main Street Singers (a kind of folked-up Partridge Family in which only one member from the original line-up survives) contrasted with the down-home rootsiness of The Folksmen ("It wasn’t retro then, but it’s retro now") and the artistic angst of Mitch and Mickey (played by co-writer Eugene Levy and the shamefully underrated Catherine O’Hara). The performances are well-judged as ever, with Fred Willard’s washed-up promoter and sometime sitcom star equalling the comic brilliance of his commentator in Best In Show, Jennifer Coolidge hilarious as a clueless investor, and O’Hara and Levy bringing some genuine emotional weight to their relationship without sacrificing the laughs. With the ensemble cast now well-attuned to the nature of the project, it’s no wonder many of the jokes – apparently the result of collective improvisation – hit home, or that the spoof folk-music is as amusingly accurate as Spinal Tap’s cod-rock.

But, despite all this polish, the structure of A Mighty Wind is so unimaginatively cribbed from Best in Show and Waiting for Guffman, there’s a feeling of déjà vu which deflates the energy and humour which distinguished the earlier movies. There’s also, in the treatment of the Mitch and Mickey subplot, a virtual abandoning of mockumentary principles, whereby the story is simply shown rather than implied or alluded to. As their suppressed romantic urges are dramatically rekindled in A Mighty Wind’s closing moments, the style becomes so transparent that we could be watching a straightforward fiction film. This might be considered appropriate given reality TV’s blurring of boundaries between documentary and soap opera, merely mockumentary keeping pace with the genre it satirises, but it arguably also suggests that Guest is beginning to feel dramatically restricted by his own format, and that a new direction is as in need for him as it is for his audience.