Scooter24 - 10:31 pm on Feb 7, 2013 (gmt 0)
In fact I just did that (discovering in the process the webmaster tools, quite interesting). Maybe I found the problem: 7 messages since the beginning of January about ncrease in authorization permission errors
"... Google detected a significant increase in the number of URLs we were blocked from crawling due to authorization permission errors. ..."
Turns out that Googlebot disregarded my robots.txt at the end of December and hit a bot trap, triggering an entry into my .htaccess deny from list. As a result Googlebot was 403-blocked from crawling my pages.
Over the course of January more and more pages became inaccessible to Google. I think I've now fixed the problem.