Trespassing is not illegal as such, but permission is needed.
You have a right to remove people on your land after warning and using reasonable force.
In addition you can sue for any loss and damage (stress) caused, together with an injunction to stop them.
I would get a Solicitor to write and ask them to stop and ask for compensation
You need to write and set out your losses and request compensation within 14 days or say you will go to Court within 14 days. You should make sure you send this signed delivery and keep a copy.
If they do not compensate you then you can issue proceedings in the County Court. You can either do this online at: www.moneyclaim.gov.uk or by completing form N1 http://s3-eu-west-1.amazonaws.com/hmctsformfinder/n1-eng.pdf and take it to your local County Court.
The Court will then issue a claim which a copy will be sent to the Defendant who will have a limited time to defend it, if not you can enter Judgment and enforce.
If the claim is for £10,000 or less it will be a small claim so you will not need legal representation. Over this value you would need representation for trial.
Can I clarify anything for you about this today please?