Sacha wrote:
Try the giant rhubarb, the inedible one! Gunnera manicata
http://tinyurl.com/2rd8mr

Some Gunnera Manicatas would look great, Sacha. But how deeply do the
roots grow? They're not evergreen, are they? They'ld be something that
the frost would turn to "cabbage", wouldn't they?

I've been mooching about in Google this morning, and until you suggested
Gunneras it seemed that ornamental grass is the only safe option. Stuff
I've been reading this morning has made me start to worry about the
roots of three large ornamental cherries on the edge of my
leach--plain/soakaway.

Eddy.