Where does the law stand on dog on dog attacks?

I've just heard today of an incident locally where a German Shepherd dog attacked and killed a Papillon. The German Shepherd was with its guardian but not on the lead [or under control]. The Papillon was being walked on a lead by its guardian.
The police were contacted, but have advised that they cannot do anything until the dog has killed three dogs? Is this correct, please?
Thank you
Nicki, by email