This plant is waist height and has variegated thistle-like leaves. It was growing in the allotment but my mum has dug it up and put it in a pot. In 40 years of gardening she has never seen one of these. Does anyone know?
your Mum's plant is the Milk Thistle (Silybum marianum (L.) Gaertn.). It's a reasonably frequent casual often coming up from bird-seed or as a wool alien and has been present in this country for a very long time as a weed of disturbed places. The flowers are purple usually in thistle-like heads up to 4cm across but it is the striking marbled foliage that is most distinctive.
Thanks Dr Fred! I thought it looked familiar! I'm surprised that it hasn't turned up in the allotment before, and even more surprised that my plant-mad parents didn't know what it was. I have forwarded your message.