Before going further, spend some time reviewing Mastering the Spelling Checker.

Turn off "automatically detect language." This is a setting that applies to all Word documents. http://office.microsoft.com/en-us/ou...010244823.aspx It should not be causing your problem since you only have one language enabled, but It doesn't hurt to turn it off.

Select all the text in your document and make sure it is set to the same proofing language, i.e. English UK. The proofing language setting is something that applies to each character in your document. Pasting something from a different document that has words in a different language setting can "infect" your document with a different proofing language.