Have just stumbled across these drums, created initially in 2000.
As a drummer myself, and often the butt of 'musicians' jokes it is great to find something rhythmic as well as carrying some sort of melody. I dont know if they have appeared in any form at Greenbelt yet, but would make a perfect insturment for meditation, or as part of an ambient chamber ensemble.
Getting hold of one may be a bit problematic though- check here
check them out at Hang Blog