There is one thing that has never been clear in all of these discussions about foreign travel. Some people seem to go everywhere and never get stopped. Others get stopped, even in Europe, where we're supposed to have free movement. I've heard people stating that green notices have been issued against them, but if that was the case, why did the PPU let that person travel in the first place? If they were so concerned about that person's risk, then why not tell them, to their face, not to go?
" SO who pose a real threat to children should be stopped but those of us who are not, and never have been a threat to children, who be allowed to enter Vietnam on good merit."
I think the problem is that the police don't tell the other country anything. All the immigration people see is a notification saying 'known to UK authorities'. No details about the offence or the persons' risk. They then have to make a judgement call, about whether to admit them, based on no information. This accounts for why some people get sent back and others don't. In all these discussions, people are looking for a clear answer, when there isn't one. It's entirely dependent on the immigration officer and how they feel, at the time.