Could forum members please identify this intruder

in Plants
A few months ago, a plant appeared in one of my borders, which I can't ignore any longer as it is beginning to over-compete with it's neighbours. It's leaves somewhat resemble Comfrey, but there are no flowers. The fully-grown leaves are about 16" long and 6" across at the widest point. It has formed a mound about 3' across and 1' high.



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Now I have to figure out the best way of getting rid of the new clump