"David E. Ross" wrote in message
If the clay is treated with gypsum to make it more porous and if the top
foot of clay is tilled with new top soil, the interface between the top
soil and the clay subsoil will be blurred. Clay is often rich in
nutrients other than nitrogen. With careful use of fertilizer and with
infrequent but deep watering, plant roots will grow down below that
blurred interface into the clay.

Given the amount of rain the UK has had over the past summer, watering is
probably not really needed. In fact it's possible that this area could end
up as I suggested - like a bog garden.