Speaking as his new film hit the cinemas, Mr Mullan said: "I still think we can do it, but we have just a year. I still think we can turn around that 15 to 18% of floating voters. I don't trust the polls."
But he fears a "dark arts campaign" by Westminster could derail a Yes vote. "At the moment I don't think we're losing the propaganda battle but they definitely have the resources and the organisation and we will find it all out in 30 years hence.
"I'm not being a conspiracist, but they'll be sitting down every day and they'll have a team paid and employed to have one task and one task only. Put spanners in the works as often as possible on a daily basis. At the moment that seems to be working and I think we have to counter that propaganda campaign."
Mr Mullan, who in the past lent his support to the Scottish Socialist Party, said he wasn't sure how artists could support the call for independence in their work.
Mr Mullan's film, based on the Proclaimers musical, is now on general release.
l For a full interview with Peter Mullan read today's Herald Magazine.