Hi Plantnut, definitely marestail. Be aware that it will regrow from the smallest piece of root, they are black and very difficult to see amongst the compost or soil. We inherited it in our garden and it is quite simply a devil to deal with. We asked at the RHS help desk at Tatton Park and they shrugged there shoulders and said best luck.
I've been living with it here for 10 years and barely notice it. i pull it out when I see it but apart from cause some extra weeding it doesn't really do any harm.
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