Further to the above, I've discovered that the OSD for the space left on the sd card displays the second partition when using the numerical value (displayed at the top of the screen, by default). This is great.
But, the bar that goes across the bottom of the screen displays the boot partition's size, which isn't useful since that partition doesn't change - nothing is written to it.
I figure that if a large sd card is being used with two partitions (boot and images), all space indicators should reflect the space left on the images partition.