The excavated base is a mix of chalk and gravelly firm sand containing flint gravel with chalk lines. It was not quite the continuous bed of chalk we were hoping for! The main chalk area was in the back right corner of the photo.
In the front right corner the topsoil was much deeper than elsewhere (towards the back there is as little as 100mm in places), and was still present at the final excavated level. We think that the chalk is about 100mm below this level, so we will need to dig down further and build the level back up.