Thanks so much - feeling embarrassed now! But I remembered that I have a Synology NAS c/w Docker, and the Roon Extension Manager etc was in there. I was able to stop one of the 2 from running, and that has cleared up a load of inconsistencies / clashes.
If I select a KEF LSX wireless speaker with Internet Radio as the Alarm output, I am unable to set either the alarm Volume or the Transition Type. But if I select a Sonos Amp instead, I am able to set both the Volume and the Transition Type.
Both devices show up under Settings / Audio as “Roon Tested” devices (“via KEF streaming” and “via Sonos streaming”). Both are set as Volume Control: Device volume within Roon.
If volume control and fading are not offered by Alarm Clock then volume control is not provided by the Roon API for the output device. Can you control the KEF volume via the Roon Remote?