Archie Leach (Cary Grant)'s grandfather was John Leach b. 1842 in Frocester, a parish of Wheatenhurst Gloucestershire. On freebmd.org you can see the birth registration as 'male Leach', no mother's maiden name as they didn't register that at that time. Later census records show that he was the illegitimate son of Ann Leach who married George Bristow in 1844. They lived in Bristol, and you can see John Leach on the 1851 and 1861 census with the Bristow name or John Leach Bristow. John's marriage record in 1865 states his father is 'George Leach', clearly a mistake of the registrar who likely was told the father's name was George and the registrar assumed the Leach part.
Ann Leach was also born in Frocester, to William and Ann Leach. William Leach was born in Stroud Gloucestershire. Most of the Leach family census records with sibs and all are shown on Wikitree.
https://www.wikitree.com/wiki/Leach-1874