Wishing to set the file manager as Dolphin I looked at the options available under the KDE System Settings.
Following a recent update Dolphin had been replaced by Konqueror as the default file manager in KDE.
In KDE’s System Settings select Default Applications. For me this was shown in the second row – Workspace Appearance and Behaviour.
In the left menu click on File Manager to show a list of optional file managers.
From the the radio button list change the selection from Konqueror to Dolphin. Click on Apply at the bottom right corner.
