It could be one of the willowherbs, but it's hard to be sure without flowers. Either keep it until it flowers and then we can ID it for you, or pull it out. It might die back over the winter and reappear in spring.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
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"We don't stop playing because we grow old; we grow old because we stop playing." - George Bernard Shaw