Can anyone kindly identify this weed?

http://tinyurl.com/7wtkbg9

It grows about 3ft to 4ft tall eventually, and is invasive. It has very
deep tuberous roots. It is very hard to kill off, because its roots go very
deep. It is becoming a problem down here in South West Dorset.

Sorry, I can't remember what the flowers look like. I always attack it
before it ever gets to the flowering stage!

Many thanks,

Jim