Thanks for your quick reply, it wasn't a profile issue as such but your advice led me down the correct path.
I checked profile path rights (all ok) and so created a test profile and found passwords to be saving normally on it.
I'm using a Vivaldi account to sync my passwords and bookmarks etc, so I created a brand new profile for myself and re-synced to that with my Vivaldi account - no luck, same issue.
Next I removed and re-installed the browser completely, signed back in and hey presto, it's working again! However...after the browser re-install I set to re-enabling my extensions, one of which belongs to 1Password.
I re-enabled 1Password and signed back into the service and the problem re-appeared.
I then removed the 1Password extension and Vivaldi saved passwords are behaving normally again.
So that seems to be what is causing the issue but quite why I have no idea. I've used both Vivaldi and 1Password for some time and never had a problem, indeed I have Vivaldi installed to my personal Mac with 1Password extensions installed and that's working perfectly.
In any case I don't really need the 1Password extension installed to my Windows PC Vivaldi, so I've left it removed.
Thanks for your insight!
Matt