Audible.co.uk are offering a free 14 day trial when you sign up for an account with them. I chose the 2-book a month account, which means I get two freebies, then once I'd downloaded my two books, cancelled my account, very easily.http://www.audible.co.uk/aduk/site/offers.jsp
That's the link, but it automatically chooses the 1-book account for you, so make sure you change to the 2-book account to get the best of the deal.
The offer runs out at 11.59pm tonight
edited to add - get £4 if you go through Quidco too - thanks to the user who mentioned this, wish I'd have checked!