Thank you so much for all the replies - it's really good of you all.

In order: Mike, thank you, I copied what "David" from another post did, and created a Google site. I hope that you can now see the picture.

Jake: Himalayan Balsam: oddly enough, it keyed out as that originally, but I didn't think Himalayan Balsam had such red stems.

NOT Rosebay willowberb (far too easy!) as that has green stems, and the leaves aren't in pairs. (I did mention that they were in pairs, didn't I?)

Pam, definitely not Herb Robert, nothing like it: oh, you hadn't read the bit about being so tall. Sorry, yes, it's very sturdy and tall, certainly not a geranium.

Kay, the leaves are not hairy, and neither is the stem: sorry again, I should have made that clear. Can you see the picture? Shiny, rhubarb-red stems, and very bright red central vein on the undersides of the leaves. Purple loosestrife is tempting, but I'm fairly certain I'd recognise that when I saw it, and again, it doesn't seem to have the very bright red stem and central vein underneath the leaf.

Stewart, thank you, not Rosebay (which as Kay comments, we all know isn't a real willowherb at all) but I'm edging towards Epilobium Montanum (broad-leaved willowherb) which Rose has as having red stems, but I can't find a single pic on the internet that shows red stems.

(*tears hair out*)

Rachel