A documentary about a UK punk band, some of whose members have learning disabilities, has its premiere next week at the South by Southwest (SXSW) music and film festival in Austin, Texas. The film follows Heavy Load, who have been together for 12 years, as they make the transition from playing disabled club nights to performing at mainstream rock venues.

Shot over two years, the documentary captures the highs and lows of life on the road for the five-strong Brighton-based band. The film's director, Jerry Rothwell, contacted the band after reading about them in a Mencap newsletter.

Rothwell films Heavy Load recording their debut album The Queen Mother is Dead and captures their first mainstream gig in a pub in Seaford, West Sussex, where they win over an initially sceptical crowd with their repertoire of punky cover versions of pop songs as well as their own material. There's also footage of the highlight of the band's career so far, playing to a large festival audience at last year's Wychwood Music Festival in Cheltenham - on the same bill as the Levellers and Badly Drawn Boy.

While documentary makers can be accused of being intrusive or exploitative, drummer Michael White, one of the three band members with a learning disability, is positive about the process: "Being in the film has made us more relaxed. We hope it'll make us famous."

As the film progresses, it charts the increasing confidence and ambition of the band. Conferences with care coordinators are replaced by meetings with record company executives.

Heavy Load are excited about the upcoming exposure at the SXSW festival. "Getting the film shown in America will mean we will reach a bigger audience," White says. "I've been going to day centres for 28 years, but what I've always wanted to do is to be in the music business."

Rothwell dismisses any idea of the documentary being sentimental about people with learning disabilities: "It's not about triumph over tragedy. It's about the joys of being in a rock band. Filming these guys, I realised they had something that is missing from so many people's lives - happiness."

· Heavy Load: A Film About Happiness will be shown at the SXSW festival on Monday

· Listen to the band's track Stay Up Late