(2-part post):
The government website that deals with this question isn't particularly helpful.
Quote:
"Someone has parked on my land, what can I do?
As the vehicle is on private land, it is for you (or the owner of the land) to resolve the issue.
The following suggestions may help you:
*Make local enquiries to see if you can trace the driver or get information about when the vehicle was left.
*You may be able to arrange for its removal by contacting your local authority. There may be a charge for this service which you could claim from the owner if he or she can be traced.
*Seek legal advice from your solicitor or from your local Citizens' Advice Bureau.
*Contact your local police if you have further evidence or information which suggests there is something suspicious about the vehicle."
Source:
http://www.crimereduction.gov.uk/faq7.htm
I suggest putting up a sign stating that the land is private and that a charge will be made for parking. (I'm not suggesting that you spend a fortune on having a sign professionally made. Type something out on your PC, get it laminated and display it - or preferably several copies of it - prominently). Leave a note on the windsreen of the car and/or drop a note through the letterbox of the driver (if you know who/where this is). Keep a record of when and where you displayed the signs and when you placed a note on the windscreen.