They're very rare - I'd be surprised if it were really a Norwich or cross. Can you post the listing? They're lovely little terriers - not too dog aggressive, but spunky. I wanted one but could never find a breeder with available puppies - they have years long waiting lists ...
Like any terrier, they're require more work off leash than, say, a herding breed (bred to work WITH you). Terriers were meant to be independent, but many excel at off leash sports when trained well.