Seen today at "Le Jardin des Plantes" of Rouen, FRANCE. Zone 7. The photo of the flower is not very good but gives some idea. Thanks for your help.
I think you're on the right track with Commelinaceae - but I haven't been able to find a perfect match after some searching.
Wonderful! Bravo Lila and many thanks. You are right. The plant is in a shade position and in a sheltered place. We hope to get seeds from it. JP