Click 'Get Shopping' and enter the code PP0811 when you are on the Basket/Checkout page.
Discount valid until 31st of August 2011.
My affiliate link:
http://www.bookdepository.co.uk/pp0811?a_aid=offers
Mod: Original expiry is 31-08-2011, however as of 07-09-2011, code still seems to work.
Before ordering check the book price on both the .co.uk and .com websites, if you find your book is cheaper on the .COM website, you can still get a discount until 5th August, but it takes a bit longer as you need to refer a friend and wait until you are emailed a discount code, see here:
http://www.ozbargain.com.au/node/46876