I know this has been highlighted before. But I've just spotted it again whilst browsing Amazon.
Basically you need to set up an Amazon Wedding List and when your 'guests' have spent £250 or more on gifts on your Wedding List. You will then (within 35 days of meeting these conditions) receive an email confirming that the promotional credit (£50) has been added to your Amazon account.
Sounds pretty easy right? It's going to be perfect for anybody getting married and thinking about asking their guests for I dunno, a PlayStation 4, a new AV receiver or a new TV.
Bob on. I'll link to the T&Cs below.