Thanks guys, but after reading all the ideas I looked carefully at the "Sun Virtual Box" panel which appears when you start VB, clicked the "hard drive" section, and in the "settings" panel I clicked "hard drive" again. Now I already had another virtual HD from another experiment, and clicking on that offered me the option of "attaching" it to the other HD. So I did, and lo & behold, on restarting the virtual machine I have an extra "E" partition.
So there you are folks - just create an extra HD, give it whatever size you feel like, attach it, restart your virtual machine and enjoy the new space!
Thanks for the ideas that got me there.