Contact a solicitor. They will use the powers of the law to help you. They may go through the courts and use ‘Part 55 of the Civil Procedure Rules' ('CPR') to evict the trespassers.
Your solicitor will have to contact the County Court - although in some circumstances they may go straight to the High Court; especially if there's a risk of public disturbance.
To start a claim, your solicitor will need to file claim forms with the court. They can download them from www.justice.gov.uk/forms or get them from the court itself.
You will also need to submit a witness statement which will usually have to include proof that you're the owner of the land. You may have to get these from the Land Registry - but your solicitor should be able to advise you on exactly what information you'll need.
Once you have submitted the claim forms, the court will then fix a date for the hearing.
The unauthorised occupants will then need to be served with official court papers. Your solicitor will arrange this for you. If there are safety concerns, your solicitor can contact the police who will accompany them when the documents are served.
If the unauthorised occupants are named, then they must be served personally. If the defendants are unknown then the notice will have to be attached to the main door or some other part of the land so that it is clearly visible.
If you can, it is worth sealing the papers in a see-through plastic wallet. This will mean that it's still visible - and it will protect the documents from bad weather.
The unauthorised occupants must be served with these documents no less than two days before the hearing date (if it is on other land).
Once the court hearing has taken place and an order for the trespassers to leave has been made, hopefully they will voluntarily leave the land.
If they do not, your solicitor will have to get a warrant or 'writ of possession' (but they may well apply for this at the same time as the original order to save time later). Your solicitor will then need to ask the County Court Bailiff or High Court Enforcement Officer to enforce.
Where there are a large number of trespassers and police help might be needed, then your solicitor will need to contact the County Court Bailiff or High Court Enforcement Officer before the initial court hearing to give them advance warning that police assistance may well be needed.