Ok thanks for all the helpful replies. Had the plot about 6 years now, a big improvement over the years obviously, been using gypsum, poultry pellets, rotted manure, fresh manure, cardboard, and trenches for beans using uncooked kitchen waste etc, but still got some areas that are difficult to plant seeds in. It's great in some places, but hard work in others, especially after a dry spell like now. Maybe I'm digging up clay from underneath, but a no dig plot is a bit of a problem, as I can't get enough manure/mulch to cover the sort of depth you need for that. Perhaps I'll try again in small patches and see how it goes.