Because I think that comment conversations get lost among posts, I’m going to respond to comments here

1. Yes, a raised bed might be the only option, but I understand that the thing about comfrey is that the roots go really deep and tap into the nutrients that can only be found in the subsoil and then these nutrients are then returned to your veg beds when you used the leaves as a mulch/compost.  Soooo….I’ve discovered that the large rock thing I’ve found is actually a lump of concrete and it’s surrounded by lots of smaller pieces and ‘grains’ of cement.  I don’t know how deep the cement goes, but it’s easy to clear and the concrete lump appears to break up when it’s banged very hard with a spade.  I’m going to borrow my Dad’s sledgehammer to make breaking it up easier (and safe-guard  my spade that is in danger of being broken!)  and hopefully then the comfrey roots will be able to reach past the crap to the good stuff deeper down (if there is any!).  I think I’m going to start a campaign for builders on new-builds to not make a horrid rubble heap of the garden!

2. Green manure is plants that you sow solely to nourish the soil - legumes that fix nitrogen for example, or plants that you cut down and dig in when they’re very leafy.  They’re meant to keep the ground covered between crops which is better for the soil, and some improve the structure because of the way their roots grow.