It looks very much like Horseradish. If it ever flowers with a slightly smaller white version of Oil Seed Rape flowers then you have it.

Thanks Fractal. I just lurve horseradish!

It will have long thick roots called thongs I believe. You need to dig up either the clump or part to get these roots out. I'm sure someone with more experience can add to this.

I was thinking a Rheum, but horseradish would fit, too.

I did think of a Rumex but decided on Horseradish as this seemed to match closely to images of it I have compared it too. Difficult perhaps without a scale but that is my educated guess.