Simon,
Sorry for the delayed response. Not sure why it's not using the Open Graph tags. When you change the Open Graph tags, it can take Facebook a few days to see the update. Until then, it will use the 'old' Open Graph tag, if set, or it will guess what the best information is to use. You can use the URL Debug Tool to see what Facebook sees, and it also updates their cache immediately instead of having to wait:
developers.facebook.com/tools/debug
Try that and see if it improves the post. Let us know how it goes!
Alex