Thanks for your replies.
I'll have a look at smbclient.
Despite all the stuff I can do, networking is and always has been a mystery to me, I seem to have a mental block when it comes to trying to get my head round it.
I've checked on my wife's PC, Win10, and my Kubuntu shared folder is visible and writeable into from her machine.
I'm dual booting with Kubuntu and Win10 and we can see/use shared folders each way using Windows. As far as I know Win10 doesn't use Homegroup any more.
I tried the Add Network Folder option but couldn't get it to work, which may well be down to my not entering the info correctly.
Just tried Xububtu in Virtualbox and the shares are there in the Thunar file manager as they are with Gnome and Mate.
UPDATE:
I managed to get the Add Network Folder option to work and can now access the share folder on my wife's PC. Seems a clunky way to do it, compared to Gnome etc, but at least it's a step forward.
I still can't find a way to access the hdd connected to our ASUS AC-68 router by USB 3.0. There's stuff online which involves editing /etc/fstab -
https://www.haidongji.com/2017/12/05...-ac68u-router/
Is this safe? I don't want to screw up fstab.