The warning about not boiling with the cork in isn't just nannying!
I was using a KK around New Year, and the mate I was out with asked 'Won't it boil quicker with the cork in'
'Dunno, i've never tried it, but it's not a good idea'
In went the cork.
Out came the cork.
Into the snow went his face. Luckily no lasting damage done.
If you put the cork in while you're using the kettle, it will swell to give you a tight fit, then the pressure behind it overcomes the friction, and it does come out with a proper bang and a gush.