I keep seeing a road sign that I can't find in the Highway Code. I'll do my best to describe it in the hope that someone can enlighten me, please. It is a black flattened diamond shape (not filled in, just a black line) on an orange background. There's one nearby that is shown to the right of a green A1 sign, for instance.
If a major road has to be closed for any reason, the necessary diversions have been planned in advance. So, rather than have to put up hundreds of diversion signs, at lots of junctions, the Highways Agency (or County Council, as appropriate) can simply put up signs at the point where the road is closed, stating (for example) "For Nottingham follow <diamond symbol>. For Retford follow <square symbol>".