I've been checking our Howick Hall as I know you visit there. I can't find anything about the plants they grow except they have picked up seeds from around the world! I will bet that China or somewhere close like Cambodia or Vietnam is where it came from, that's where plant hunters hunt for plants.
Crug Farm Plants might have it, they go to these places ..
http://www.crug-farm.co.uk/
If you're ever back there when seed has made, you know what to do?
