Found at the Leeds Ikea although I presume it's national as there was a few duvet sales in the sale and lots of stock.
The price is with an Ikea family card which is giving 10% of all sale items. The card is free and you can sign up at the cashout desk if you need to.
If not the sale price is £10.
The other sizes of duvet covers were available and a little cheaper I believe - I should have checked properly but I was all Ikea'd out at this point :)
Hope it helps someone.