Nothing to do directly with your question, booksy, which Ethel and NORMAN have answered very succinctly. I also note that you are not trying to say that you in some way exempt from producing your documents because you were stopped on private land.
However, beware of your understanding (at least for motoring matters) of �private land�. The law describes areas to which the public has �unfettered access� as �public roads� for the purposes of the various road traffic acts. This is regardless of who owns or maintains them. Car parks at supermarkets and the like are good examples of this. Unless the area where you were followed is gated and has access restricted in some way it would almost certainly be interpreted as a public road for motoring purposes, and all the motoring laws will apply.