I saw this plant growing at the side of a path next to a house wall in Yorkshire, it looks like it just seeded itself there. I don't have a photo but did a basic drawing. It was about a metre and a half tall by a metre wide. The flower spikes about 30cm and the leaves quite large ( 15-20cm long). Hope you can tell from the picture!! The flower spike was mainly large green seeds with some flowers towards the top.
your plant is a Pokeweed - Phytolacca - almost certainly P. acinosa Roxb. the Indian Pokeweed as this is the most commonly grown and escaping (through being bird sown) species.
That's great thank you. My husband is a horticulturist but he'd never seen one before. Will the green seeds turn black? As the photos I found on google all seem to show black seeds.
Yes. Well, a very dark purple actually.
The 'lacca' part of the name indicates a red dye.
Note: all parts of the plant are toxic, though some parts can be eaten if suitably prepared - I wouln't risk it!