lucy24 - 5:52 pm on Dec 10, 2012 (gmt 0)
:: belated "D'oh!" moment ::
The originating site must link to some specific page on your site. So that's where you need to check for referers. And then match those up with requests for the /thanks page to see which ones went the whole way from Other Site to a completed purchase.
I thought the search phrase carried through to every page