Since the location was named on the order it appears that police may have somehow got inside information about the site of the party. The directions to the site were not made public until around 10pm in the evening when information was put on answerphones.
Many free outdoor parties have taken place over the past few years with the police showing very little interest. Sites are usually carefully picked so they won't disturb anyone and people and organisers are generally pretty good at getting sites cleared up in the morning. Many of those attending have a higher level of environmental awareness than more mainstream society. There is almost never any violence at such events.
So its unusual for the police to take this step where not only police vehicles but a police helicopter was used. Allocating these kinds of resources to stop a few people dancing in the woods may seem completely bonkers compared to any sane perspective of reality, especially when one considers all the other kinds of real crime taking place in the cities of Manchester and Sheffield on a booze fueled Saturday night.
But this is an increasingly repressive society. It's a society that is making singing in public a criminal act and where you can be branded a terrorist and arrested for simply wearing the wrong T-shirt.
But despite Plod's best efforts and very expensive flying machine the party did finally get going at around 4am in a squatted building. Police still attempted to turn the music off and it was only after they were barricaded out of the building that the heinous crime of dancing with other people could take place.
It remains to be seen if this was part of a wider clampdown on free parties or perhaps, more likely, it was just the police responding to some overzealous party advertising around Manchester. With more free parties planned in the summer only time will tell.