Hi again
The full entry Rosie99 found in Elsdon reads (from an Ancestral Indexes transcription):
Ann Potts born 18 July 1804 & baptised 18 July 1804 Illegitimate daughter of William Potts, Labourer by Ann Robson.
As the child was baptised on the day of her birth I checked for a burial but could not see one under either Potts or Robson. My transcript goes up to 1812.
You mention that you have been 'given' the parentage of John Robson & Isabel Foster. This seems to come from an Ovingham baptism:
Ann Robson born 21 Jan & bap 9 Apr 1809 5th child of John Robson of this parish by his wife Isabel Foster.
These dates are consistent with the dob you originally provided - what is your source for this date?
A dob of 1809 is also reasonably consistent with a 66 year old Ann Charleton dying in 1876 in NZ.
How do you know that your Ann Robson was a) born in Ovingham & b) an 'only daughter'?
Do you mean only child or only daughter?
I got the Ovingham bap from the Bishop's Transcripts (note these only relate to Church of England events, not non-conformist baptisms, although non-con couples may have had no choice but to marry in the C of E if Scotland was not an option between 1754-1837 & burials of non-cons are likely to be included, as the churchyard may have been the only burial ground.
You can search the images of the Bishop's Transcripts for some C of E parishes (including Ovingham) for free online at:
https://www.familysearch.org/Scroll down & click on Europe & then choose the Bishop's Transcripts for Durham & NBL. These have not been indexed and so will not show up in the results for any searches you do on the website, though you may find other useful info.
You need to scroll through as you would a film. Please note that there are gaps in all the BTs, sometimes just for a day or so, which can be hard to spot, sometimes for weeks, months and years.
Hope some of this helps

,
Matilda
P.S. This announcement from the "Daily Southern Cross" seems to provide a bit more confirmation that the Charletons who keep hitting the news in Kawhia may be your family:
On 21 Jan 1860, at the Mission Station, Kawhia, by ...Mr R. Nazer to Jane, youngest daughter of George Charleton, farmer, Kawhia