Hi Ady, I just had a look at the replies. I'd tried to add the text box but couldn't get it to work for some reason!
My way is much simpler but this may only work with ms2010...
Go to insert, page number, page margins, pick 'border, left' under 'plain number'. This will insert your page number where it should be but the rotation will be following the text rather than the page if that makes sense. But if you then rotate the text, you're sorted!
Hope this helps!