UPDATE: Thank you so much for all of your advice, concern, support and kind words.
The incident was reported to the Police, the council, the dog warden, the Community support officers, the safer neighbourhood team, and The RSPCA.
We have been told by pretty much everyone that there is little that can be done, unless we know where the man lives.
We know that he lives around the Mayow Park Area, as a number of our neighbours have seen him walking his dogs in the park.
On the night this happened the owner said he was shocked and that it was "the most brutal thing he had ever seen" and that his dog had never done anything like this before. The next day he to my house with a bunch of flowers and said that his dog has attacked cats and dog before and that he would now have the dog "put down" on saturday at Battersea Dogs Home. The dog warden has check to see if there was any record of the dog at Battersea and there is not.
The dog warden has told me that I can push for a prosecution but that he will only get a £75 pound fine and I will have to take him to court and give a statement in order to force his to pay, they also said that they may have to exhume the cat's body from the grave to get this done. She also warned me that there might be repercussions if I pursue this as the man knows where I live. If only the dog had been on a lead this would not have happened.
The police have told me that because the attack didn't involve a human they can't do anything. The community police officers told me that there have been a number of dog attack in the Mayow Park Area in the last few weeks and months, involving dogs and cats and one involving a child.
I have started and online petition to insist that dogs are kept on leads in residential areas, please sign it, forward to anyone you can and post it wherever you can. Maybe it will help stop anyone else from ending up in this situation.
Thank you so much again for your help, advice and support, I really appreciate it at this sad time for my family. The loss of a dearly loved pet really is like losing a member of your family, and I will miss my cat so much. It was such a senseless waste of live and such a horrific death. Hopefully we can stop this from happening to anyone else.