Just thought I'd post some info regarding trespass as it crops up here during debates/advice about DCAs. Just like many things, people need to do some research themselves but for what it's worth, here's a couple of links that may assist you, together with a few cut and paste sections that I found interesting.
This link is just a general answer: https://www.askthe.police.uk/content/Q56.htmTrespass to land is a civil wrong and as such the police have no jurisdiction.
Under common law, the landowner has a right to re-entry on the land. However, the ejection of the trespasser is fraught with danger for the landowner. Initially, the landowner should ask the occupier to leave the land and if he/she does then all is well. However, the problems start if he/she refuses to leave the land.
The owner of the land could commit several criminal offences if he forcibly removes the trespasser and his/her property from the land. The best and safest course of action is to obtain a court order, which if breached may then turn into a criminal matter.
If the police do attend an incident such as this, they are merely there as observers for any possible criminal offences committed by either party. The police cannot assist in the removal of the trespassers or their property from the land in question. The police do have some powers against larger groups of occupiers if damage has been caused.
Next is what I believe to be the relevant act: http://www.legislation.gov.uk/ukpga/1994/33/part/V
Criminal Justice and Public Order Act 1994
Offence of aggravated trespass..
(1)A person commits the offence of aggravated trespass
if he trespasses on land [F20in the open air] and, in relation to any lawful activity which persons are engaging in or are about to engage in on that or adjoining land [F21in the open air] , does there anything which is intended by him to have the effect— .
(a)of intimidating those persons or any of them so as to deter them or any of them from engaging in that activity, .
(b)of obstructing that activity, or .
(c)of disrupting that activity.
[F23A constable in uniform who reasonably suspects that a person is committing an offence under this section may arrest him without a warrant.]
69 Powers to remove persons committing or participating in aggravated trespass..
(1)If the senior police officer present at the scene reasonably believes— .
(a)that a person is committing, has committed or intends to commit the offence of aggravated trespass on land [F24in the open air] ; or .
(b)that two or more persons are trespassing on land [F25in the open air] and are present there with the common purpose of intimidating persons so as to deter them from engaging in a lawful activity or of obstructing or disrupting a lawful activity, .
he may direct that person or (as the case may be) those persons (or any of them) to leave the land.
(2)A direction under subsection (1) above, if not communicated to the persons referred to in subsection (1) by the police officer giving the direction, may be communicated to them by any constable at the scene. .
(3)If a person knowing that a direction under subsection (1) above has been given which applies to him— .
(a)fails to leave the land as soon as practicable, or .
(b)having left again enters the land as a trespasser within the period of three months beginning with the day on which the direction was given, .
he commits an offence and is liable on summary conviction to imprisonment for a term not exceeding three months or a fine not exceeding level 4 on the standard scale, or both.
(4)In proceedings for an offence under subsection (3) it is a defence for the accused to show— .
(a)that he was not trespassing on the land, or .
(b)that he had a reasonable excuse for failing to leave the land as soon as practicable or, as the case may be, for again entering the land as a trespasser. .
[F26A constable in uniform who reasonably suspects that a person is committing an offence under this section may arrest him without a warrant.]
Hope that this is of assistance.