Solved Exotic looking plant thriving in London suburb

I have now had a year's experience with this interesting plant, so thanks again for the identification.

It turns out that the plants are either male or female. Both are attractive but the male is more attractive -- the one in the picture is male. The female isn't as tall, the stems don't arch so elegantly and the flowers are yellower -- she's like a Golden Rod on steroids.

Unfortunately you can't tell which sex you have until it's in flower.

Both male and female are very good value plants -- they give you something interesting to look at from April till October.

Dead easy to grow, dig a hole anywhere, keep it moist till it's established -- it establishes rapidly.

 

I don't believe this is true. In it's second year the flower form has changed to become more like the one in the picture, I'm pleased to say. It presumably just needed to get its feet down.

It's a fabulous plant.

 

Many thanks for reporting back to everyone.
That was a hard one to get correctly named.
Interesting.