If you try following the upgrade link for Yosemite from the Apple.com website you’ll be disappointed I’m afraid. The installer isn’t ready for download. I’m hearing that it is a global issue, so don’t worry those in the US Store haven’t got a head start on the rest of us!
UPDATE: 22:12 GMT
Yosemite is showing up in the Mac App Store but still not downloading, so it is likely minutes away from being available.
UPDATE: 23:30 GMT
In case you didn’t already know the installer is now available … enjoy!
(Blog title updated to reflect the change.)