Sorry to hear this news, not nice at all.
I am always alert to off lead dogs, and plan my walks accordingly.

The only dogs that I can think of that frequent the area which I see are a white elderly staf walked off lead by an elderly gent.
And two staf type dogs usually exercised on the grass area between 24 Sunderland (the flats) and the flats / garages on the corner by the south circ.

All usually off lead, but the two latter dogs are far more active and interested in other dogs.

Hope the responsible persons are found.

PS I am not making allegations towards any of the above dogs, just pointing out they are local.

PPS, as an after thought, there is another dog which is muzzled but usually on a lead, which is walked by a guy in his 30s. Usually well controlled all the same.