Hi it is enchanters nightshade. those white 'roots' are rhizomes and they will grow again from a small piece. It is a British woodland native and it is a lovely plant but it is /can be a thug. I have it in my wild area but wage war in the neater beds.

That's good to know as I've been waging a war against it for years and always wondered what it was!

Many thanks for your answer. I always seem to get it in two particular areas of the garden. I always dig it up and hopefully as long as I do that I can keep it under control.
I am in the process of digging up Japanese Anenomes as they just seem to get everywhere. Little did I know when I planted them in different areas of the garden how thuggish they would become.

they both like the dampish conditions .
I keep digging both out too :o)