According to my watch sunset to sunrise is about 6 hours 50 minutes. I believe the last time I camped on lowland, farmland it was about this same time of year on a tour of the Lakes. It was getting quite dark when we pitched up, three small tents. We left shortly after sunrise.
It was in a livestock field without livestock in it, they were a couple of fields away. We were also in a dip, below the main field height just above a stream with trees on about 3 sides iirc. It was a horrible midge fest so we went straight to sleep. We all got up and packed in maybe the half hour before sunrise. We had packed up just after sunrise and moved on to have breakfast and a brew later on.
Basically we used a bit of common sense in that we were pitched significantly away from a likely direction a farmer would pass, very discrete location and we were pretty much packed up and mobile when it was daylight. No road nearby to be seen by passing farmers or farming community members. Even far away from gates accessing the field.
There was no campsite on our route for several miles and hiking not cycling meant we couldn't easily travel to a campsite. Unlike the slurry camper who it seems was easily noticed and was very close to commercial campsites. The argument of the site being too expensive doesn't give you any rights to camp on farmland so that's a non-argument. If you can't or don't want to pay for what you want to do then don't do it. In this slurry campers case if he cycled past commercial campsites because he didn't want to pay for a legal stay well that's just another point against him imho.
Still, no need to spray slurry. Two wrongs don't make a right.
On one side there's an inconsiderate, self entitled cheapskate and on the other is a farmer with questionable judgement and possibly anger or control issues. Nobody comes out great here.
The only positive here is if even just one wildcamper learns about and uses the principle of pitch up late and move on early to avoid conflict in future. And another farmer thinks twice before direct action against a wildcamper.