so sorry to hear about the terrible news, how awful for you.
Firstly dont bother with the rspca, its not a welfare issue so they will do nothing, the police may log a crime number but with little evidence there is nothing they can do, but would be worth at least having a chat with someone down the station.
Secondly get incontact with your local dog warden (if you have one), and the council, it IS an offense to allow a dog to roam the streets, but as it happened on public property the current law is a little shakey regarding dog offenses on private property, but it would have had to cross public property to get into yours so it has committed the offense of being dangerously out of control in a public place as is stated in the 1991 dangerous dogs act. The original act before it was amended does cover private property but this would involve solicitors etc so think the council would be your best bet.
Whether the dog is the same one you think it is, there is a dog loose on the streets and it needs to be dealt with so at least you can pursue this and maybe the council will find the dogs owner, a loose dog could cause a road accident (as well as all sorts of hazzards and environmental health issues). Maybe they will have better fencing, if its rented property you could tackle the landlord, if its owned then you may have a battle on your hands.
so sorry this happened