I didn't see that episode, but its available on line - the one transmitted on 6th September is Episode 26 , so just google BBC gardener's world episode 26 and it should come up for you to watch. I just had a quick skim through, but didn't find the bit you're talking about and I don't really want to watch the whole episode to find it!

Eupatorium maculatum? Google it and see if it's the one you mean.

That's it, Jimmytheone! Thank you. And thank you Bamboo for the info on how to watch a missed programme if I need to. Now, the blurb on it says it is vigorous. Does that mean it would be a thug?? These days it's got to be something that I can control without too much trouble!

Certainly can make a big clump, if it likes your soil. Ours was over 4 feet in diameter in just a few years.

Thanks Owdboggy. I can probably cope with a clump.