iPad and iCloud oddity

I have an iCloud account which I currently use to sync contacts and

It also holds Keynote presentations.

I have an iMac (OS 10.7.4), an iPad (iOS 5.1.1), an iPod Touch 2nd
generation (iOS 4.2.1) - everything syncs as I expect although it took a
little effort to get the iPod to sync :-)

I can also access iCloud via Firefox from a Windows PC and the iMac.

Find My iPhone is installed and working on all 4 devices. The iPod, iPad
and iMac are shown on all devices.

Except ... when I try to access iCloud via Safari from the iPad, it
diverts to the 'welcome page and wants me to set up iCloud on this
device, although in setting, it is already set up and every works.

I've deleted Safari cookies, data and history and removed all website
data. Without effect - the iPad Calendar and Contact sync with iCloud, I
just can't access iCloud via Safari.

I didn't originally have this problem - I can't be sure but I think it
originated when the iPad was updated to iOS 5.1.1

Any suugestions as to how I get Safari on the iPad to let me access