Word Word spell check not picking up US spellings

[Nickyfrance]

I have recently installed Microsoft Word 2007 and am having a problem with the spell checker. I have found other discussions of problems with the Word 2007 spell checker but mine is very specific.
When I set the default language in a document to UK EN (as opposed to US EN), the spell checker picks up words such as 'labour/labor' but does not pick up any words using a 'z' instead of an 's', e.g. organisation/organization. These always used to be highlighted in Word 2003 so why aren't they now?

Thanks for any advice.
Nicky.