At last I got the phone screen to display on my PC !
After many hours messing about, what worked for me was to log out of my local account on the PC and log back in directly with my Microsoft account - which I thought I has already done since I was logged in to my Outlook account with my Hotmail account, and to the Edge browser and indeed to the Your Phone app - but apparently not yet on a 'Networked' account on the PC !!
So I used the Sign In section on the Start panel at the left of the main screen to sign back in again with the same Hotmail address but selecting 'Microsoft account' NOT local account - and bingo ! All bits of the Your Phone app now work !! Although it did take quite a bit of perseverance with authorisation codes, QR codes and what not to get there. Phew !
Of course I've also had to reconfigure my 'new' login account on the PC and for tidiness removed the old one. But all is grist to the mill as they say . . ..