I had a similar issue when I purchased my Golf. You would either need to go into the original VW dealership that carried out the service or post the book to them for them to stamp it up.
I had to post mine to them, with a covering letter. I would telephone them first and speak to someone in servicing, get a name and post to them specifically, so they can deal with it.
I know it sounds long, but I even contacted VW head office and they told me the same thing. I had to send my service book to 3 different VW dealerships to obtain all the stamps in the end!
Let us know how you get on.