Good point Robin and I sam a lot of it when the digger removed trenches for the greenhouse. There is a six to ten inch layer of dark loam over pale, dense clay, with few stones but variable amounts of gravel introduced by the earlier market garden as paths etc. Also some areas of concrete where they had greenhouses, not too many thankfully.
Essentially a fertile soil as long as it is not driven over in wet weather! Even after the recent rain, walking causes squelching in some areas.
Incidentally Lower Farm was clay over limestone, no chalk.