Thanks Pete and Chris. I didn't wait that long actually, only took five attempts and moved on before I started to look weird. But that was helped by the fact that Regent street is always extremely busy and there was no shortage of potential subjects to line up the shots.
Also I composed using the LCD, I think it helps a lot for this kind of picture.
True, lining that up with a standard RF (or even the EVF) would have been nigh-on impossible. A real bonus that the colour schemes of the guy and the mannequin / store front match so well.