Thank you all for your responses, many great ideas so far! I have not tested the pH but as the soil is very clayish I assume it is rather on the alkaline side of the scale.
Phytolacca americana It is tall, has pretty leaves and fruit.. but the fruit is toxic. It grew in the woods next to our house, when I was a child, and we would smash the berries to make dye or writing ink. I'm still alive.
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Gunnera? (Needs to be very moist soil)
Ornamental rhubarb? (also needs to be moist and v fertile
Umbrella leaf?
Astilboides tabularis?
Symphytum x uplandicum variegatum (grows to 1 metre).