Below are some easy to understand guidelines

Our interpretation is given in the following examples

You hear a smash, someone is breaking into or has just broken into a car across the street, you see them running away - Call 999, you meet the criteria below. The same could be said for a suspicious activity, if you feel that something is amiss in that regard call 999.  If you see someone spray painting a wall -999

Contrasted with

You go out in the morning to find your car broken into and the criminals long gone call 01702 431212, then your insurance company.  Or you find graffiti on a wall and it is dry.

To assist the police in dealing with genuine emergency calls the following guidelines are given:

Where a caller has an honestly held belief that any of the circumstances below apply, then the caller is justified in using the 999 system.

Where there is a threat to life.
Where there is a serious offence or incident taking place, including a serious road collision.
Where there is a likelihood of an early arrest or a suspect is at scene or nearby.
Where there is violence towards persons or property occurring.
Where important evidence relating to the above is likely to be lost.

Remember that if you dial 999 and your call is not an emergency you may prevent a genuine emergency call being answered. These are examples of situations which are not emergencies and do not justify use of the 999 service:

I've locked myself out of the house and need a locksmith.
There's water coming through my ceiling.
My car's broken down, can you come out.

A helpful leaflet on who is responsible is here