William is on the 1901 Census living in Droylesden near Manchester. I presume you already have this information.
Herbert Mansell 13Manchester, Lancashire, England SonDroylsdenLancashire Joseph Mansell 6Manchester, Lancashire, England SonDroylsdenLancashire Mary Mansell 45Ireland WifeDroylsdenLancashire Mary Mansell 18Ardwick, Lancashire, England DaughterDroylsdenLancashire Matilda Mansell 23Ireland DaughterDroylsdenLancashire William Mansell 54Corfu, Turkey HeadDroylsdenLancashire William Mansell 16Manchester, Lancashire, England SonDroylsdenLancashire
All the children are born in Lancashire so he should be on earlier censuses
Have you checked the Government Overseas BMD records - accessible on line through 1837 Online for a fee (£5 for 55 searches)