Buyers from within the EU:
VAT payable at 17.5% on just the buyer's premium (NOT the hammer price)
Buyers from outside the EU:
VAT payable at 17.5% on hammer price and buyer's premium. If a buyer, having registered under a non-EU address, decides that an item is not to be exported from the EU, then he/she should advise Christie's to this effect immediately.
L. Synge, Art of Embroidery, Woodbridge, 2001, pl. 154.