I was once out on a job with another engineer. On the way back, he asked me if we could call in at his parents' house, near Derby where he was brought up. John spoke with a totally normal Derby accent, until the moment he walked through his parent's door- when he adopted a thick Glasgow accent. After a pleasant chat with his parents, and the inevitable tea and scones, we left- and he immediately reverted to Derby. I mentioned this to him- he told me he hadn't even noticed he was doing it.

McColl/ Miller's parents were Scottish, and it's quite probable that he spoke that way at home.