Shiro, to clarify, especially as others might just skim this, the points I made aren't "valid" - they are from the Government website - http://www.gov.uk - it's the law.

You're quite right in that reporting a dog just for being big and off the lead is OTT. But any dog that is perceived to be a danger - "makes someone worried that it might injure them" - could be in trouble. Even a yappy little nyaff.

If the dog is on the lead and clearly under control then it would be difficult to argue it was dangerous. But even if it's on a lead, if the owner is clearly struggling then that's a problem.