I think the purple loosestrifes (Lythrum salicaria) do grow pretty tall. My photo probably isn't the best example of one as it's grown within a pot in the pond and the flowers weren't all over the plant at that point. I'll have to try to get a photo of the growth that is starting as I'm sure it matches up pretty well. Certainly looks like a type of Lythrum anyway.
That is not a label at the back of the clump? Left side there are three or four short bits of old cut off flower stalk sticking up, and something square and white in the middle of them.
Edited to add suggestions Lythrum or Lysimachia. I think lysimachia mostly come in yellow or white flowers? I know the leaves are various forms and shapes and leaves do not seem right. Apart from the groundcover one which does have rounded leaves and looks a bit like your plant obviously not tall enough. But Lythrum are the same family apparently. Might help.
That was a bit of wing off a garden ornamental bird long gone sorry
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