The Thursday post should be visible with these as a combined Search term ... "ken duncan" + uluru + baloney - but I cannot see it on Google. Very frustrating. Especially when I could see that post via Google Search approx. 24 hours ago.
There is no technical reason I can find as to why a specific post or page on your site would not be visible to Google for indexing or why it would not appear in search results. Everything appears to be configured properly.
Your posts are also appearing as expected in our site indexes and global tag pages here on WordPress.com.
Indeed, several of your posts about the park are showing in Google's index, but not the one in particular you are mentioning here. You can see this by running this search:
The most recent post in that result is from the 25th, so it's possible that the index has been rolled back or that the initial search caught it and Google does something weird to it. Unfortunately, Google can be a bit of a mystery sometimes—to us as well.
Have you tried enabling and using Google Webmaster Tools? They can be very valuable for learning more about how Google sees your site and parses it, including the frequency of indexing. You can learn more about how to enable it for your WordPress.com blog here: http://en.support.wordpress.com/webmaster-tools/