I later changed my category structure, redistributed the forums between them. Because prefixes are per forum not per category this messed up the prefixes.
I changed the prefixes to match correctly but the display does not update.
Tried updating caches on threads no difference.
At the moment the prefixes are displaying the wrong way round!
Also I notice that on the Prefix 2nd tab Applicable Nodes: the list of nodes has not updated from my restructuring - it still shows the old names for the categories. Confusing.