Your first port of call might be better made to the local council's dog warden service. The council may have (and many have) adopted into policy Dog Control legislation which means that they can enforce notices upon the owner. They may also be aware of the people already and be able to tie up your info with existing details. They can of course also advise further, increase patrols providing they have GOT a dog warden as some councils no longer do, their legal obligation only to be to have someone tasked with the duty of collecting strays. This need not be a dog warden per se but is often another Environmental Health staff member (often pest control).
The Police's response can be varied. Some might go all out, others might be less proactive. By all means report but don't bank on a huge amount of action (although, wrongly, they can go the other way completely and apply a possibly misinterpreted use of the DDA).
As someone who is involved in dog welfare I find it important to point out that there is no proven correlation between dog on dog attacks and dog on human aggression. That's not of course to say that it doesn't happen but that the two do not go hand in hand as a matter of course.
All I can add at this stage is that I'm sorry that this has happened to you, hope that pooch is none the worst for it and reassure you that the majority of GSD are not like this. I own two and often work with several others.
It would probably be beneficial for you to seek out a training class or similar whereby you can introduce your dog to others, particularly GSD and similar large breeds, under supervised, professional guidance, in order that you can quickly work on avoiding any long term fears and possible fear aggression towards other dogs as a result of this horrible experience.