There's an entry in garden.org's database under Dragon Fruit (Hylocereus undatus), which seems to fit your identification of this plant. Why not ask to have your photo moved there? I see that you already have at least one photo of this plant in that gallery already. Not sure how this one ended up here, but it's fixable. Pity to have such a great photo 'orphaned' like this. Good luck. - Elizabeth

Thanks Elizabeth. Saw your comment and have moved the photos from Hybrid Orchid Cactus (Epiphyllum 'Undatus').

Thank you both for moving the pic to the correct place.
Hadn't realized I had made a blunder.
Will do better in future!