Konabunnt – the way I read it was:
The court, at first instance found that, although the commanding police officer at the scene on the day felt there was an imminent risk of a breach of the peace which would justify the containment, in the opinion of the court, in hindsight, that breach of the peace was not as imminent has the police thought, which made the containment illegal.
The court of appeal found that this was the incorrenct test, and that the correct test was whether the belief of the police officer in charge on the day that there was an imminent risk of a breach of the peace was a reasonable one for him to come to at the time, based on the information he had at the time – and if this was a reasonable belief, then the containment was lewful.
I note also that they clearly made the point in the judgement that the other, more violent, demonstration up the road was lawfully contained, and that nobody on either side had challenged this.