The premium is the Auction houses cut from the deal.
From what I can make out it's 15% upto £30,000 and 10% after, thats if it's the same as cars.
On top of that you have to pay the normal VAT (value added tax) 17.5% on the the amount of the premium.
For example , if an item sells for £100, you would have to pay £115 plus the VAT on the extra £15 = total £117.62.5
Sam.