How can I cancel my iCloud Storage plan?

When I upgraded my iPhone 4 to iOS 5 and it turned into a "brick" I was forced to use iCloud to backup and restore and it kept complaining that I had 5.9 gb which is over the 5.0Gb free storage provided, so I purchased an upgrade - in a dazed and confused state as my iPhone was a "brick" I must have done this twice to recover my iPhone.

Result is that I now have an additional 20Gb Storage Plan.

Nice email saying that I can cancel it within 15 days, but not where or how to do this. I spoke to Apple iPhone support who said its another division, try iTunes support website and I should be able to email and cancel - but not seen any email address.

Tried iCloud Express Lane but they want £20 for an incident ticket which is about what I paid for the 20gb Storage Plan!

Problem overall is that I use an iMac (PPC not Intel) on Mac OS 10.5.8 and although I have upgraded to iTunes 10.5, I am finding that WiFi sync or iCloud do not work unless you are on Mac OS 10.6 (needs Intel) or greater.

So I want to cancel my iCloud storage plan. Please can someone help me.