You are correct but there`s a snag. If the document has been archived or moved to another computer the creation date will be wrong. For example, I`ve just opened the properties of one of my own documents from 2008 and the creation date is in May 2010 when I transferred to a new computer. The modified date looks about right and last accessed date was a year later when I used that document as a template for the following year.
Not particularly conclusive.
Isn`t there a NHS reference number on the document that might pin down the date? Is it a "standard" letter or one that would have had to have been written specifically for the recipient? How can the NHS prove it was sent after it was created?