I had never heard of "Ghostwatch" until I read the accompanying article. But it appears it had much the same effect in Britain as Orson Welles' "War of the Worlds" did in America back in 1938. The special was about finding haunted places in Britain and back in those pre-Internet days, viewers couldn't check to see that it was just a made-up drama. 

Apparently, some of the effects and stories were portrayed so realistically, that people believed them and the BBC was inundated with an estimated one million phone calls from around the country while it was running. But with only a few operators on duty, many of those calls never got answered, only spreading the falsehood about its 'reality.'

The special has never been broadcast again in the U.K. and appears to be permanently banned there because of the fallout. (According to Wikipedia, it did turn up on the Canadian cable channel scream in 2004, so some here could have seen it.) But that doesn't mean you can't still see it. The Internet Archive has the full 90 minute show available for free online and you can watch it here. 

The TV show so terrifying it left children with PTSD and has never been allowed to air again in 30 years

It's hard to believe so many could be taken in by this, but it was a different time. Now, if you want to be truly frightened, all you have to do is turn on any nightly news show. And unfortunately, those stories are all too real!