mbovenka - you may be on to something !
Because having gotten fed up & unplugged, I then felt braver, plugged in again, opened Calibnre again - to see a blank Library...worrying, however, I then realised it was showing s"send to Device" (still nothing showing on My Computer).
So I knocked out and reopened Calibre a couple of times - and Library re-appeared books and all !
I tried to send one title to "the Device" - the Opus remember - and it told me I already had it on device, install anyway ? Said yes, and there it was on the Opus.
Plugged in T1, all well there.
Have since sent 4 titles from my Calibre Library, and just now downloaded a new title from Gutengberg OK, and it's gone to the Opus OK.
Which is now showing on My Computer.
So I was definitely confused - and I think the PC was as well !