Having looked it up. As far as I can see, the police would have investigated to determine the type of Assault that was caused. It sounds very much like it fell under common assault which is only prosecuted on complaint.
This is the definition of Common Assault.
This is any act by which a person intentionally or recklessly causes another to apprehend immediate unlawful violence. Such an act must be with the intent being calculated in that persons mind to cause apprehension or fear in the mind of the victim. Therefore, where there is no intent, there will be not be an assault, UNLESS, that the person who assaulted another, ( and it was conclusive by way of evidence), that the person was indeed reckless as to the other person would in all probability have indeed apprehended that immediate unlawful violence would be used.
Therefore Clarkson may have been guilty of Common Assault but the police will only action this on receipt of a formal complaint. This also applies to offences such as vandalism, petty theft and other less serious offences.