I don't think the film has all that much to do with Frank Sidebottom, beyond the name and the head. It's more a take on more troubled outsider geniuses like Captain Beefheart and Daniel Johnston. As far as I can tell Chris Sievey was maybe quite eccentric and probably a bit frustrating, but Sidebottom was a piece of art he was doing and not an artifact of mental illness like the film's Frank.

I believe the documentary was the origin of this article.