As much composted material (leaf mould, composted animal manure, spent mushroom compost, your own home made garden compost, whatever you can get hold of) and, if its that bad, some bags of horticultural grit too, added as often as possible (particularly the composts...)

Thank you, I'll try it!

If you are short of home made compost, Autumn leaves, chopped with a lawn mower if you can, and /or grass mowings can be spread over the soil in autumn and left over winter. By spring much of it will have disappeared and the rest can be dug in if there's access between the plants. if not just leave it there. Seaweed is good too but there probably isn't a lot around Dudley!
Add a little fertiliser in spring to compensate for any loss of nitrogen due to the rotting down of the mulch.

Every year you do this will improve your soil a bit more.

Also check the pH of the soil. If below 6.5, add dolomitic lime. If above 7, add gypsum and epsom salts. That shouldn't replace the organic matter, but will help it to work better and faster.