If you go to “Settings” → “General” → “Folders” and then under Favorites mark the “Junk-email-Folder” checkbox, you will have a unified junk-mail folder that holds the junk-mail for all your accounts.
Empty that via right-click and you’re done.
Appreciate the suggestion but it’s not what I wanted as this adds further “clutter” by way of favourites at the top of the folders tree, making it look untidy to say the least.
I’ll wait until hopefully they can add a menu option directly.