I've spent a couple of hours on the internet, trying to pin down which thistle is which. I'm pretty sure Kenker is the common purple flowered type and it is now bubbling in a pan on the stove. It was meant to take 30 minutes and it's been going for over an hour and is still pretty chewy. If we enjoy eating it, I'll post the recipe tomorrow.
Kenker - Purple - http://en.wikipedia.org/wiki/Gundelia -