Hi Rob
The IGI actually shows her father as John on the 1850 marriage entry, which seems to confirm the death cert information. There were a number of Marshalls baptised in Ampthill at around the right time with a father John, but no Hannah.
Fortunately they seem to be in Ampthill in 1841
HO107/006-03 folio 9A
John Marshall 45 Registrar notary office Not born in county
Mary Ann Marshall 35 Not born in county
Harriet Marshall 15 Not born in county
Hannah Marshall 15 Not born in county
Adelaide Marshall 8 Not born in county
Hilare Marshall 4 Born in county
Patience Marshall 5mths Born in county
Picking Adelaide as the least common name, on the IGI there is an Adelaide West Marshall baptised at Chatham, Kent on 17 Aug 1831, parents John and Mary Ann (controlled extract so reliable). But no other Marshalls baptised in this parish.
It's too early in the morning! A simpler way would be to look for them in 1851 where birthplaces are listed!
1851
HO107/1754 folio 220B
Church St, Ampthill
John Marshall head marr 59 Clerk of Ampthill Poor Law Union, Actuary of Ampthill Savings Bank; Registrar of births and deaths. Born Southwark St Johns
Mary Ann Marshall wife 47 b Strood Kent
Harriet Marshall dau unmarr 27 b Canterbury Kent
Adelaide Marshall dau unm 18 b Chatham Kent
Hilare Marshall dau 14 scholar b Ampthill
Patience Marshall dau 10 scholar b Ampthill
George Robert Savory visitor unmarr 25 Schoolmaster b Islington Middlesex
I cannot see Hannah in 1851 or subsequently but assume she would have been born in Kent, possibly Canterbury ie after Harriet and before the family moved to Chatham where Adelaide was born.
Never assume anything! Just found what appears to be William and Hannah in 1851
HO107/1754 folio 462
Shelton, Marston Moretaine, Beds (3-4 miles from Ampthill)
William Negus head marr 31 Master butcher b Husborne Crawley Beds
Hana Negus wife 24 born East Indies
Mary Ann Elizabeth Negus dau 7 scholar b Bedford
John Randolph Negus son 3mths b Marston
Hana has been transcribed as Maria, but it could be either. Her age is right, her husband is right, her son has the name you mentioned as being on her death certificate as her father. There's just the problem of William possibly being a widower when they married. But I can't see an obvious first marriage or the burial in Beds of anyone of the right age between 1844-50.
If you get her marriage cert, which can be done online, the witnesses may link her definitely to the 1841/51 family, as well as her father's occupation. At least you know that the entry is correct if he was the registrar! And it should state if William was a widower.
East Indies is a bit different, and possibly explains why you couldn't find her in Beds!
Hope this helps
Regards
David