This is Himalayan Balsam, a plant which was brought into Britain in the 19th century because the gentry thought it looked exotic. It's now rampant, an invasive weed that causes havoc alongside railway lines in particular. I love its flowers, though, and at this time of year, of course, it is near the end of its cycle until even more of it sprouts up next Spring. It's great for attracting insects, too, but they are now disappearing as Autumn approaches.
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Photo from Terrycanon's gallery.