In reply to abr1966:
An interesting one as just walking over Dartmoor would not (in the main) be civil trespass as most of Dartmoor is CROW land, but to wild camp in England, Wales and Northern Ireland you do need the landowners consent, it will fall under the trespass laws unless stated differently.
Civil Trespass is just essentially get off my land, so if you have not damaged, taken something, or it's on land which has further protection such as Railtrack, Military, etc all the owner or representative can do is ask you to leave.
If you then refuse or then return it can become Aggravated Trespass, which is a criminal offence and you can be arrested for that. Again if the owner can prove that you have damaged something, more likely if you were camping in a crop field or using stones off a wall for a campfire, you could get charged with criminal damage.
I have spent too much time trespassing in derelict buildings and structures over the years, so coming equipped can also be an option, but very unlikely wild camping as you can justify the Swiss Army camping knife. In the main it is the land owner's representatives who are the most aggressive, the land owner you can talk to and the police even if they may not be clear of the laws initially will find out and often have fallen in my favour of just civil trespass.
I suppose it all comes down to common sense, like it always has. If you are low key you will slip under the radar. If it is a large event you do ask the landowner, if you are asked to move on fair enough try and have a chat, but if the owner is adamant don't get arsey and move on, it saves repercussions for other people.
Post edited at 17:19