Thanks all for your thoughts. It's funny as this is the only new page I've added to the site in the last 3-4 years (it's been up for about 10 months now) and no, none of the other pages have been regularly updated.
Upon closer investigation it looks like Google's cache is Aug 2nd whereas all of the other pages on the site are Aug 13-14th.
This page is equally well linked to internally so this to me suggests that SOMETHING about this page offends Google to the point that it refuses to crawl it regularly like it does most of my site. There is no coding error that could explain this behaviour.