I have connections (very distantly mind you) to the George James Underwood who was one of the murderers. He was eventually sentenced and charged with manslaughter for shooting John Dunkley and was transported to Tasmania. The killing took place on 6th Oct 1840 in Yardley Hastings.
I have quite a bit of information on James, but there's no family connection for me for Dunkley (not that I know of) so I can't help you there.