Raggy, when you made your purchase this year, did you opt for the 'free one-day delivery' option? If so, you probably didn't notice that it's tied to a free trial of Prime and, at the end of the trial period, they can debit your a/c with £49 - extract from Amazon site below:-

"Amazon Prime Free Trial FREE One-Day Delivery is available to Amazon Prime members. To join, select Yes, I want FREE One-Day Delivery with a free trial of Amazon Prime and confirm your Amazon Prime free trial sign-up during checkout.

Amazon Prime members enjoy:
Unlimited FREE One-Day Delivery, with no minimum purchase requirement
Express Delivery for just £4.49 per item
Shared benefits for members of the same household

Important: Your credit card will NOT be charged when you start your free trial or if you cancel during the trial period. If you're happy with Amazon Prime, do nothing. At the end of the free trial, your membership will automatically upgrade to a full year for £49."

Maybe you unwittingly signed up by opting for one-day free delivery?