Signing up to Vodafone phone and broadband, the guy in the Vodafone shop said that if you double your data, use it all within the month, you can buy another block and so on.

I'm sure that you can only double your data once per month? I said that to him but he was adamant you can, and I can't really see anything black and white on Vodafone's site, unless I'm not looking in the right place.