Trespass to land in most instances is a civil matter, and as such the police do not have the power to assist. Initially, the landowner should ask the trespasser to leave the land and if he/she does then all is well. If he/she refuses to leave the land then you will need to consider taking civil action. It could be dangerous for the landowner to try to remove the trespasser themselves.
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, can then become 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. However the police do have some removal powers against larger groups of occupiers.
Any damage done by a trespasser, or use of threatening, abusive or insulting behaviour towards the occupier, may amount to a criminal offence and should be reported to the police on 101.
Trespass is very complex and guidance should be sought from a solicitor where appropriate.