It looks like a white comfrey. If I'm right, you want to root it out a bit sharpish, they can take over your garden. Makes good plantfood, though.

I agree its one of the comfreys.

Thank you for the replies. Yes, I think I need to get it out rather quickly. It's already in quite a few places in the garden, even coming up in the path where there's the odd space. Thank you all for your help, it's greatly appreciated.