hermantj post in this forum how to get Cortana Working it works great in the uk
Good news, got a fix :
Set your language and region and speech to us-english.
Then go check the updates. There is one (it's the language update

). Install it. Enjoy Cortana
EDIT : seems to NOT work. Sorry
EDIT2 : it works. BUT in order to make it work, you need to launch it the first time from the app (not the search button).
A Hard reset seems to work too
this should be a link to the thread he posted in
http://forums.windowscentral.com/cortana/274732-2.htm#post2459054
p.s I have also discovered that i needed to remove the us language and add it again, also make sure you click it and click move up so its listed first in the list of languages