I would be grateful if someone could assist with identifying the plant in the attached photo for me? We only moved into our present house last winter, so I wasn't aware of what was going to appear in the garden in the Spring, however, this plant appeared at the beginning of August and has now grown to about 18" in height.

We've potted it at the moment, as to date we haven't been able to identify it in any of our books and subsequently don't know how big it may grow? Could anyone please assist?

Many thanks