If the dog is a danger to others it needs to be 'muzzled' whilst in public! The owners of the dog are also legally responsible for the dog 1 metre inside there perimiter! (think it's one metre - could be further) this is to protect children and those amongst us that are unable to read any written signs that may be in postion! which cannot also work the wrong way for the owner incidently!

Ring the CAB, or the local dog warden and press on with looking at what options are open to you would be my advice!
regards
DT