Its differant in each part of the country, some have lead laws, some dont. (since it sounds like the other dog wasnt on a lead). You need to call your local council and see what they say. They would have all the information you need. I dont think the police would be able to do anything about it,but theres a department at the council that deals with that type of situations.
I had a similiar situation, when I was walking our doggies, a big dog came out of nowhere and jumped on our 13 yr old greyhound,pinning her down and going for her (luckily the other dog had a muzzle on), but unfortantley in this area,theres no lead law and there was nothing we could do about it
Hope your doggie feels better.