I will be pleased to assist with your request today.
This is a difficult area as there is no automatic right to remove vehicles parked on private land. If you do so, without following the rules, you run the risk of committing a criminal offence as it is an offence to immobilise, move or restrict the movement of a motor vehicle in a way which would prevent a person (who is entitled to remove that vehicle) from removing the vehicle concerned.
In order to avoid the risk of prosecution you need to ensure that appropriate notices are posted on site to confirm that unauthorised vehicles are subject to removal. You should also ensure that you have provided written notice to the offending business owner to confirm that he has no right to use that area to park vehicles and that he only has rights to pass and re-pass to obtain access to his own property. It would also be good practice to send a similar letter to other users pointing out that there have been instances of unauthorised vehicles parking on your land and that you have had to take these steps to prevent that.
In your letter to the business owner I would suggest you allow him a reasonable period of time to remove his vehicle (normally not less than 14 days), failing which you intend to refer the matter to removal agents who will take steps to remove the vehicle to secure storage. In that way you avoid running the risk of a police complaint by intentionally depriving the owner of access to the vehicle. The removal agents will likely charge for storage which the owner will have to pay if they wish to retrieve their vehicle at a later stage.
You should also write to the DVLA and complete form V888. They will provide you with details of the registered keeper and you should also ensure that a copy of your notice letter is sent to them.
Your local police and/or Council will be able to assist you with contact details for private vehicle removal companies. In some areas they have the capacity to provide those services themselves.
The first step though is to ensure that clear notices are posted within the car park such that they are clearly visible on entry and from within the car park. I would suggest the following wording -
"This is a private car park
Any unauthorised vehicles may be removed by vehicle removal agents. You run the risk of incurring additional storage costs if your vehicle is removed in these circumstances. In the event that your vehicle has been removed please contact _________________ regarding recovery of your vehicle."
In the blanked area you should insert contact details for the removal agent you choose to use.
I hope that this is of assistance and I wish you luck with your parking issues.