Many thanks to Nigel Aspdin for bringing my attention to a small notice in the 26th February, 1873 edition of The Derby Mercury which confirms that the Slaters had indeed taken over Horsley Castle Quarry (and Coxbench quarry) by this date. So, it seems almost certain that the Slaters would have supplied stone from their quarry for the dressing of the Margaret Street façades. Here is the wording of the notice:
COXBENCH & HORSLEY CASTLE STONE QUARRIES, NEAR DERBY.
W.H. and J. SLATER, having taken and reopened the above Quarries, beg to intimate that they are now prepared to SUPPLY the best quality of STONE for every description of BUILDING PURPOSES, either in large random blocks or cut to any dimension.
Address: W.H. and J. SLATER, BUILDERS, DERBY.
For a scan of the original notice visit Nigel Aspdin’s Denby brickworks page here.