"Sacha" wrote ...

It's a native perennial wild flower of unshaded wet ground (marshes, pond
&
stream edges, ditches, etc) and is seen all over the country in suitable
habitat. Probably some local to you up on the moor.
Having never cultivated it I'm unsure of it's suitability for
introduction
to the garden scene but it does flower late in the season.

Thanks, Bob. It's extraordinary that we've never seen it but I shall now
open my eyes wider.

http://www.aphotoflora.com/page19.html
Might help you locate some.

--
Regards
Bob
In Runnymede, 17 miles West of London