Andrew Mawson presented the following explanation :

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?

No particular experience of this, however....

Rather than the bricks flaoting up, the soil will fall through between
the bricks to fill in the voids between. Only once those voids are
full, will the soil stay on top. Might be better to grade the infill
with course rubble layers, working down to the finer material, then
finally soil.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk