I just realised that my spell checker keeps offering American spelling as a correction ie favor instead of favour. The document language was set to English (UK) so why the problem? Apparently you have to set the Proofing language, mine was set to English (US) for some strange reason.
So here is how you change it...
In Word 2010 select the following options from the Ribbon:
- Review tab > Language group > Language button > Set Proofing Language
- In the Language dialog, select the language you want to use for spell checking.
In Word 2007, Review tab > Proofing group > Set Language will accomplish the same thing.