Thanks Carl. Yes, Roon uninstalled, machine rebooted and Roon re-installed; pre-amp also power cycled. I now know that the problem is with my regular core machine that uses the USB output to the pre-amp. I authorised another machine using PC speakers and Roon worked normally on that.
I have made some progress in that files are now playing, but still not getting any sound. I suspect its something to do with the audio settings. I think I’ll stumble on the solution eventually, I just hope it doesn’t turn out to be something obvious and stupid that I’ve done .