Display one or more clear signs. (Without them the courts won't back you). They should be worded roughly as follows:
"PRIVATE CAR PARK - By using this car park you are ENTERING INTO A CONTRACT to pay the charges stated below:
Guests: Free
All other users: £80 per day (or part thereof)
Payment may be made to the proprietor at Lyndy Lou Guest House. Should payment not be received details of the registered keeper of the vehicle will be obtained from DVLA and that person billed for the sum due, plus administration costs of £20. If court action is needed to obtain payment from the driver or from the registered keeper (as permitted by Part 3 of the Protection of Freedoms Act 2012) the payment of costs will be sought from the court."
Then stick to your guns, get the relevant information from the DVLA
https://www.gov.uk/request-information-from-dvla
and pursue people through the courts if necessary. You can do it online:
https://www.moneyclaim.gov.uk/web/mcol/welcome