Q652: What should I do if I suspect my neighbour has an illegal type of dog?
You can contact your local police (via 101) or Crimestoppers on 0800 555 111 and report the information to them. Q651 has a list of the types of dog that are illegal to own in the UK.
The police will make any necessary enquiries and take appropriate action. if you are concerned over the way a dog is being treated, you may want to make enquiries with the RSPCA who will be able to offer advice.