There are a few things to consider. Are all the vehicles owned by you on your private property, or are the vehicles owned by third parties?
Do you carry out repairs to vehicles on a commercial basis? i.e. for payment in any form which could be construed as a couple of pints from a friend you have helped out. Is there any covenant on your deeds regarding parking of vehicles? If there is I am not sure that would be in the remit of a planning officer.
Another possible issue is are the vehicles parked a detriment to the local environment including a visual impact. Counter argument to that is the vehicles are part of your right to peaceful enjoyment of your own property
Now the planning officer will have to give you reasons why you need planning permission ask politely what grounds you need to apply for planning permission for and the exact regulations.
How did the planning officer know they have not moved for 28 days? Has the planning officer entered your private property and inspected your vehicles without your permission? That is a No No and possible a case of civil trespass unless he can prove that he has the legal right to enter your property and inspect the vehicles.
We had a neighbour complain about our drive and the local planning officer decided we should apply for retrospective planning permission for vehicle access road which would give the neighbour the opportunity to object. An email explaining that our drive had been in existence for 30+ years and predated any regulation that he quoted, dropped kerb had been supplied by council, gate was far enough from the road to park on prior to opening the gate, that the drive surface was porous etc. If we parked on the road it would be a traffic hazard as it is a narrow road. Therefore we were not going to be applying for retrospective planning permission.
Just be polite, ask exactly under what regulations you need to apply for retrospective planning permission. Do not be tempted to tell them to go forth etc. All that will do is get their back up which will result in more grief for you.