So here's the stack in question, a WS Drachenbau Yin-Yang progressive stack:
First flew this gorgeous stack at Stoney Cross, at winds between 2 and 8 mph. It needed 5-6mph at least to fly properly, but when it did, it did!
The black-and-white really gives it a graphically very strong appearance in the sky.
Flight was controlled and pretty slow, but I don't doubt speed (and pull) will increase when the wind goes above 10mph.
And finally, a wee bit of video, showing the stack in action:
Update: OK, one more picture then, from flying the stack at Stokes Bay today: