Thank you Judith/Ruskie/Debra/JM & Tazzie for all this info....
So possibly my John Mantell & wife were the Mr & Mrs Mantell that arrived Sydney from London on 24 Aug 1886 on the ship SUTLEJ. I've sent a research request form to Queensland Archives on how to glean more info on the late JM who died at the Benevolent Asylum, Dunwich. I await a response, but in the meantime anything else you guys can provide me would be welcome.
I've found passenger info for a George Mantell age 33 & wife Sarah 37, plus Ada 13, Lewis 11 & Gertrude 6 arrived Brisbane on 1 June 1887 on the ship Waroonga. He was a carpenter & they came from Bedfordshire. Possibly this is John Mantell's son George born 1853 Bedford. In 1861 George age 8 was home with parents & siblings. In 1871 he was age 20 born Bedford, living & working as a hairdresser in Skipton Yorkshire. On 11/3/1872 he married a Sarah Ann Bunnett in Bradford Yorks. The marriage certificate has him aged 21 a hairdresser; his father was John Mantell, hairdresser.
Via Trove - from Australian Star newspaper of 28/4/1892 a marriage on Apr 23rd 1892 at St Barnabas Chirch, George St west (Sydney?) of Charles Victor Wilson, 3rd son of Edward Wilson, of Beckenham Kent to Ada Christiana Mantell, eldest daughter of George Mantell of New Zealand.
Cheers John