I may have to cover perhaps a 1/3 acre of builders rubble that a
previous owner has used to infill a small valley (amazingly with
planning permission it being greenbelt) with soil to return it to a
semblance of normality. It will probably only be grassed over and
possibly used as paddock

People are telling me that bricks will 'float' up from below through
the soil - has anyone any experience of this phenomenon, and what is a
reasonable depth of top soil to aim for?

AWEM