Found 25th Apr 2015
Amazon Local are offering a free voucher to new members of Amazon Prime - which, of course, comes with a free trial. So once you've started your free trial, you can redeem your voucher and then cancel! I've looked through the T&Cs and, as far as I can tell, it's a) only open to new Prime members, and b) doesn't require you to pay for the full year in order for the voucher to work. Fingers crossed!