The Offence which is being discussed is being in charge of a motor vehicle whilst over the prescribed limit.
When someone is charged with this offence, they will be interviewed (when sober) to give their version of events. They then have the opportunity to put forward a statutory defence that there was no likelihood of them driving the vehicle, whilst they were over the prescribed limit. It is for the defendant to prove this point during interview.
It is also during interview, that the Police have to establish, beyond reasonable doubt, there WAS a likelihood the defendant would drive.
The example Ian gives of someone getting the coat out of the boot of the car. There would have to be more to it than that, for example, that person then getting in the driver's seat and putting the keys in the ignition. No court in the land would be able to prosecute for just getting your coat out of the boot.
With regards to the private property aspect, the Courts will have to look at each individual case, and it will depend on a number of factors, such as is the property enclosed by fencing and gates, are the general public allowed access at certain times, and so on.
In the case euro mentions, it sounds as if it's an open car park, as the Police patrol car was able to drive in. Therefore this sounds as if it has been classed as a public place.
In short, the vast majority of law abiding citizens sitting on site, in their motorhome with a bottle of wine, watching the world go by, have nothing to worry about!
Lara.