Ok, I found this after I was proceeding to put in a wish on kde bugs (hoping somebody would tell me), and found an old bug raised with the same issue.
Basically, there is no GUI function to reset kwallet.
If you open the file:
$HOME/.kde/share/apps/khtml/formcompletions
the very last line holds 'sites never to remember passwords' for -> [NonPasswordStorableSites]. Make sure konqueror and kwallet are closed, and then remove the site URL you wish to be 'unremembered' and save it.
Next time you visit the site, kwallet will now ask again if you wish to remember the details.
Phewww, that was hard work finding that out.
Nick