Hi Jill
I certainly endorse your comments about IGI coverage.
I wonder whether Mary was born in Cookham. As you know, there is no significant presence of the Payne family there. The earliest marriage is John Payne 1761[could this be Mary's brother?], and the earliest baptism is 1781.
In a quick look at the IGI baptisms for Cookham I found at least one entry for each of the years from 1735 to 1750, so it is unlikely that complete pages have been missed. Therefore if Mary is there, the transcriber either missed a single entry or miss-transcribed it. It would be too much of a coincidence for the transcriber to have missed other Paynes, so Mary would be the only Payne baptism from the start up to 1781.
It is possible that Mary had family connections in Hurst before her marriage, which may explain why she married somebody from Hurst. There is a continuous presence of the Payne/Pain(e) name in the Hurst parish register from 1589 to the present day; unfortunately, Mary was not born in Hurst. {IGI only covers Hurst baptisms up to 1607}
Good luck
Colin