Newbury St Nicolas BERRY

[JRF1974]

Great that you found Maria's baptism, thanks! Was Elizabeth referred to as Betty there too? Perhaps that James Berry was a brother of William.  As William died before 1851, his place of birth isn't specified, but he was out of county when living in Marylebone in 1841, so could also have been from Newbury.
 It seems a bit odd for those times if William and Betty only had two children.  Daughters Jane and Maria made up for it though, with 11 and 9 respectively.  Why do you think Elizabeth maiden name may be Duff?Tuff - did you find a William Berry marrying a Duff?

[Wahian]

Elizabeth was using Betty at the time of Maria's baptism.  It does seem unusual she had only 2 children and if Jane was her first, she was around 38 when she had her; that seems a little unusual too so the possibility exists this might not have been her first marriage and possibly earlier children under another surname.
The same might apply to William, so the status of either of them at their marriage might yet explain why there appears to be just two daughters. Until their marriage comes to light, we'll never be sure.

James Berry was born in the 1780s; 1841 living at Hampstead Marshall and died 2 years later. I see Jane ended up in NZ with husband Thomas Key near the end of 1856, as you said.

Thanks to Newburychap for clarification of the 1815 Census. 
Duff---found stated on a family tree, but as it also has errors on it, we have to be cautious that they got this part right and I'd give it a 1% chance they did.

[JRF1974]

Jane Berry was my 3G great grandmother. She died in Wellington NZ in 1886. Her husband/my 3x GGF Thomas Key died there in 1867.
Definitely finding William Berry and  Elizabeth's marriage would be key to finding out more, and as Elizabeth was born in Newbury it's very surprising that we can't find her marriage there, unless as you suggest, they married in a nearby parish, or some parish records are missing.

[Wahian]

While Berry doesn't seem very common in the Newbury area, it does pop up in the records of St Peter's church at Brimpton, a few miles to the east. William and Lettice Berry had 9 children baptised there between 1707 and 1724. Some seem to stay and others move away as between 1758 and 1834, five marriages appear, and not surprisingly, Berry burials are recorded too. Thus, is William coming from one of this family that moved away?   Is the surname Beare a variation of Berry?  In the mid 1700s, they were a short distance away at Crookham.