A tip for Sir David. Try to work out when he is not working. He does sign during his downtime periods, but the trick is to catch him at the right time. I had the same preprint in early 2008 and my 8x10 photo returned unsigned, so I tried again. I reasoned he would probably finish work sometime in December so he could spend some quality time with his family over the Christmas holiday and sent my 8x10 photo again on 6th Dec 2008. It came back 5 days later handsigned by the great man. See here for a scan -
http://www.cyshp.co.uk/djason.jpg