This case produced what is known as the Waterfield test[1] (incorporating the common law "ancillary power doctrine") for the limit of police authority to interfere with a person's liberty or property.

It turned out that the car was owned by Eli Waterfield and driven by his friend, Geoffrey Lynn, but police were unable to make any arrests without further evidence.

Waterfield and Lynn were charged for assaulting a constable who was in the execution of his duty contrary to the Offences against the Person Act 1861.

To come to this conclusion, the court made a key analysis of the requirements needed to show that a police officer was in the execution of his duties.

In most cases it is probably more convenient to consider what the police constable was actually doing and in particular whether such conduct was prima facie (on the face of it) an unlawful interference with a person's liberty or property.