One of our gardeners, an ex-miner, explains that many pits which were excavated on clay earth had a brickworks constructed nearby - often owned by the pit - to make use of the excavated spoil.  Presumable the Wollaton pit made use of bricks from other collieries, and the miners "borrowed" some to make use of on the allotments!...  We do also know that demolition rubble was delivered to the allotment site by the City Council during the demolition of parts of the Radford area.