You should contact your local Safer Neighbourhoods team. They could at least do a basic "knock on the door" investigation, maybe find out who the people they threw out were etc... Chances are it will come to nothing, even though it is Criminal Damage, due to the amount of paperwork and time it will take and the fact that the CPS will not even look at it as there were no witnesses! :-/
It's a sh!t state of affairs, but it's the system that we are stuck with! :-?
Hope the egg comes off the car!