Many thanks burnout.
Yes, I am synching with Opera on the mobile 'phone. Is this what's preventing the cookie data from being wiped? I would have thought deleting on one device would do so on the other.
Most of the time the sync works seamlessly (eg. for bookmarks) without my having to do anything, but sometimes it stumbles and I have to scan the QR code to update it. I've no idea why this should occur.
Appreciate your help.