Changes to the 1991 Dangerous Dogs Act came into effect on 13th May 2014.
The National Animal Welfare Trust has distilled the important information here.
Your dog must be under control in public and on your own property. Dogs that display threatening behaviour (even in the home) could land you in trouble with the police. Fines and prison sentences are set to increase for some convictions and your dog no-longer has to bite to be deemed dangerous.
The NAWT has this advice sheet regarding the new legislation.
Threatening behaviour towards a stranger or visitor can be as simple as growling, barking or lunging. Make sure your dog is under control at the door.
Landela Dogs offers 'Ding Dong' training if your dog has trouble staying calm when visitors call.