Trespass alone is a matter of civil law. This means that the police should not get involved. However, they often do attend the site and often do side with the landowner.
Trespass is entering or putting property on or remaining on land that belongs to someone else without their express or implied permission. What this means is that there should be some permission (whether it is explicit permission or implied permission, such as with with shops). If this permission does not exist, then you are trespassing.
If you have implied permission to enter somewhere, as it is with shops which are open to members of the public, then you are not trespassing until you have been asked to leave by the owner of the building or an agent (representative of the owner). In a shop, this person is often the manager. The representative should not be a police officer. If you fail to leave the land, then the land owner may take civil action against you ('sue' you).
Aggravated trespass, however, is a criminal offence. This means that the police can get involved and you can be arrested for it.
In order to be convicted of the offence of aggravated trespass, the prosecution must prove that
- You trespassed on land
- where people were engaging, or were about to engage in lawful activities (such as working)
- and you then did something (apart from the trespassing) to intentionally obstruct, disrupt, or intimidate others from carrying out those lawful activities.
In addition to this, a senior police officer has the power to order any person believed to be involved in aggravated trespass to leave the land. If the person/people refuse(s) to leave after being ordered to by police officer, or if they return to the land in question within a period of three months, this is an additional offence.
The maximum sentence is 3 months imprisonment, or a fine of £2500, or both
Sentencing starting point: really varies dependent on who you are and where you were. The sentence could be anywhere from conditional discharge to suspended sentence. First time offenders would likely receive a fine of between £200 – £300.