Let's get back to the basics
does your caravan have wheels on it?
If the answer is yes, then it is not a permanent structure as it can be moved. Suggest that if he is not happy with the position of the caravan then you are willing to reposition it to where he chooses within your garden. You can't do it right at that moment but you can assure that it will be moved to where he wants it sited within 72 hours, demonstrate you are willing to work with him and that it is a moveable structure.
You call the land your garden
Within planning is it classed as garden or is it agricultural land?
If it is a garden, great, I would ask why can't you have a temporary, mobile structure on the land? If it is agricultural, you've got a bigger fight on your hands.
Choose your battles, I wouldn't demand names as under the data protection act they won't disclose who has made the complaint and getting into that barney is a waste of time and energy as you won't win that one!
Also the enforcement officer is only doing his job and he is the one that has the power! Do not make an enemy with him because that will not help your cause.
If however, you feel the enforcement is unfair and if the officer abuses his power and comes across as intimidating then advise him that you will seek advise from a planning consultant and all further correspondance is to be through them.
If others have similar caravans, ask if your neighbours have an enforcement to remove their caravans, if not, why not?
A mobile structure does not need to be frequently moved. In my field, I have 5 temporary mobile structures. They haven't been moved for 5 years, as long as they can move either with wheels on skids attatched