Purchasing Icloud Storage Problem

Hi
I am trying to purchase additional Icloud storage
for the first time.
I keep getting an unknown error message when
I press the purchase button.
I have double checked my Itunes payment details
and all are correct.
I have no problem purchasing apps etc.
Can anyone please suggest a solution?
Thanks

 

Many thanks for your fast response Gabriel.
In the end I contacted Apple (one day left on my
warranty which I hadn't realised!)
Agent said there was a way of getting around the problem
by going into control panel/network on a pc and clicking
on the icloud icon there. I had researched a little and seen
this solution won't work with XP(I am away from home right
now and only have access to a netbook with XP and Itunes).
I argued this point with him but he insisted it would work
with XP.
He e mailed me an Apple article and lo and behold this solution
only works with Vista and W7! Grrrrrrr!
It would be nice if Apple just sorted it so I could just use the
purchase button on my Ipad!!

 

Ps.Did try purchasing from my Iphone rather than from
the Ipad.Also tried over 3g as well as wifi.
Always the same unknown error message (