Comfrey

I think the one you need is called Bocking-14, it is sterile so won;t self-seed all over the garden, so can only be grown from cuttings.  Comfrey is very vigorous, seed-raised plants will take over after a couple of years!

i wouldnt buy it, ask around your allotment, somebody will give you a few roots for free , im sure.
I know a few places were bocking 14 can be found in the wild (dont know how it got there though)  i dig up the root snap it in half  (keep half) and re-plant half.
Remind me next spring and i will send you some roots if you want?