No, it was my english that was the problem - I was rejecting those suggestions on the grounds that they were hairy, which yours most clearly wasn't.

Yours looks exactly like "the other" weedy species of Epilobium, and I'm never sure which that is, having never sat down with a key and identified it. But I noticed one yesterday in my garden, and thought "that looks exactly like the one on urg". Problem is, it only grows to about 1 foot or 18inches high. The red colouring can be a function of cultivation - eg drier soil. The one I was looking at was growing out of an old brick wall and definitely had he red colouring.

So - forgive me for suggesting this - is it possible a) that you're wrong about the 3-4ft b) that the plant you've photographed is growing close to the 3-4ft one but isn't it? I have the weedy Epilobium growing in and amongst rosebay, and when I'm weeding in early spring it's easy to get them muddled, even though they are so different.

Incidentally Fitter et al describe E montanum as "short to medium" which is up to 2ft.

I think you're going to have to let one plant of it flower - it's the only way you're going to get an id.