The link put up by DiggingDeeper looks very interesting, but Ye Gods I wish the person writing it had used paragraphs! They make text much more digestible. My (totally amateur) idea on the lack of glacial erratics on Bidston Hill
is that, being a high area, it would have been scoured clear and any deposition would have been on lower-lying areas. Smaller ones lying around might later have been used for building or walling. There is a rich variety of erratics (mostly small) eroding out of the cliffs at Thurstaston, well worth exploring.