I too hadn't realised about the £15 top up for my PAYG mobile broadband and bought a £20 top up, but when a succession of failed game patch download attempts pushed me past the 3 GB limit, it did give me a further 3 GB allowance, with the remainder showing as calling credit.
My main problem is, that due to all the failed patch downloads, I'm now getting a pop up saying "3G volume limit approaching", what exactly is this? and will it stop me from my usual use of the connections in my Global Moderating position on the Ubisoft game forums?
The 3 GB limit is ample for the use I normally need it for, it was only the problems with the failed patch downloads that led to me using so much in the first place.