I've just signed up for this (it means you get free one-day delivery for a month). As soon as you sign up, go to your Amazon Prime settings and click on "don't auto renew" or something similar and it means it won't automatically take money from your account when the free trial ends. If you want to continue it's £49 a year, which seems quite reasonable if you order a lot from them.
Thought it might be useful in the run-up to Christmas.