Hi Adam,
I don’t think this can be Chamerion angustifolium, as the leaves on that plant grow singly, in a spiral whorled manner around the stem.
Your plant looks to have opposite pairs of leaves.
It doesn’t look particularly hairy, but I think with the paired leaves and side shoots, it is showing the general growth manner of Epilobium hirsutum. But I would have liked to see a close up of both sides of a complete mature leaf, to get a better idea.
If it’s not E.hirsutum – then I’m stumped.
Regards,
Mike.