If you simply search for Mant or Mant* it should come up ( I had the same problem with my Postle ancestors - typing the full surname produced zero results!).
The entry is for James Mantell 3 December The will of James Mantell late of West Bromwich in the county of Stafford who died 6 March 1880 at West Bromwich was proved at Lichfield by Elizabeth Reeves Mantell of West Bromwich Widow the Relict one of the Executors.
While I am here on the subject of wills I note that the Probate Calendar for Charles Creighton Mantell uses the expression administration with will. Have you obtained these documents although, as I understand it and others who have contributed to the thread may correct me on this, you might simply receive the letters of administration if you apply for them.
This may be a red herring but have you researched Louisa Smith b c 1849 who would appear to have been living with Charles Creighton Mantell and his father? ( see 1921 census free index and 1939 reg). She died in 1943, her probate entry saying she was of 21 Cregoe Street, probate to Joseph Edwin Winter, retired commercial clerk.
William