Hello, I live on the edge of the Oxford clay in a very rural area between Milton Keynes and Bedford. While walking with the dogs, especially after ploughing, I regularly find large fossilised shells (Gryphae and other bivalves) and bits of ammonites etc, loads of ironstone, flint and loads of shelly limestone. There is one particular field which has lots of the type of rocks in the photograph attached. They vary in weight, even similar-sized rocks vary in weight. They are quite sharpe to touch. None of the members of my local geology groups can decide/agree what this rock/stone is. My neighbour says locally its called pudding rock, but I have a few books on fossils of the oxford clay and can't find any real match.
We thought it might be some kind of slag, as historically a lot of Hugenots settled locally to work on farms, but no-on knows if they lived near this particular field. In addition the rocks aren't really shaped like broken up slag.
Hope you can help. It's not urgent - as the rocks have probably been there for some time