The quarry sides were undamaged, however, and produced a good range of calcicoles including Aloina aloides (fruiting), Ditrichum gracile and Trichostomum crispulum.
Finally, an area of hilltop limestone outcrops to the west of the quarry (photo below) added Pseudocrossidium revolutum as well as more Ditrichum gracile. I found Dicranum bonjeanii in this area in 2011 but couldn't relocate it on Sunday.
Thanks to Peter Sturgess for suggesting the ID of Ditrichum gracile, which he'd found in the same area. I was put off the scent by the small size of the patches at Cefn Onn (only 2cm tall), but am pretty sure this is what they are.