It sat on the window sill in a glass of water for a while until we'd had a chance to be sure of the identification, then I cooked some and cautiously sampled. Yep, tasted the same. Into the soup it went! Who doesn't love free food?
Reading some more about it, it seems that the seeds too are edible and very nutritious. It's closely related to quinoa, one of the only complete proteins. The plant can become very tall - up to 3m - and as it ages, the stems become woody enough to be used as walking sticks! I think I feel another gathering trip coming on.