Thanks for the contributions.
This issue was resolved by following advice from Samsung UK support.
The fix was to:
1) Reset Smart Hub (wipes all downloaded apps)
2) Update factory shipped firmware version 1009 to latest firmware version 1025.1 (I did via USB pen drive)
3) Reset TV (Hold EXIT button on remote until message appears), then run through setup wizard as if just out of box.
4) Reinstall Apps
This firmware would normally have been automatically downloaded as part of the automated setup / out of box setup, but I skipped network configuration on day one as I wanted to hard-wire the connection rather than use Wi-Fi.
Interesting point is that YouTube never worked at all on version 1009, as well as apps did not retain app specific data. Now YouTube works perfectly, allows me to sign-in and remembers login when I close / re-open.
Also Deezer (music streaming app) now remembers login details when opened / closed.
I do not have to login to Samsung Hub.
I suspect that that login is if I want to buy Apps from the Samsung Marketplace / App Store, which I haven't needed to, as yet.