Having read a great deal about the subject - my GGGGrandfather was one of the wounded - I have mixed feelings about the film. The story it told was fairly accurate, as far as it goes. It did however ignore both the events which took place during the evening following the masacre and the attendance of people from many areas surrounding Manchester.
I was also surprised at the lack of gore portrayed during the masacre itself; the amount of which certainly does not correspond with contemoraneous accounts.
I also found some of the filming rather amusing. E.G. There were reports of attendees having drilled on the moors prior to the event. Yes, this activity was portrayed, but the filming was clearly done on high level moors rather than the low level moors which surround the Manchester catchment area.