You'd think a film that has actors lip-syncing to recordings of other people would be a device that has the potential to be annoying, even unnecessary (not to mention get old very quickly); but you get used to it after only a minute or two, and it even brings a couple different qualities to the piece throughout. Primarily it makes the line between theatricality and documentary blur, which I'd speculate may well have been one of the things that was so notable about Andrea Dunbar's seemingly raw plays themselves. (She almost couldn't exaggerate how outlandish the world she lived in could be.) More intermittently it makes it feel almost like a ghost story, and that works very well for a film so haunted by the past.
The most powerful thing about the film for me was the way it ties the whole horrific history together with one final, almost summary tragedy which reverberates back through all the different layers of pain, abuse, and social decay.