Hello,
Spotted this one as I've been transcribing those two parishes on FreeREG and I remembered the names.
Yes, it looks like they moved from Gt Oakley and had children in Beaumont from 1817 to 1830, and Rebecca was 40 when she died, which gives us a rough birth date of 1796.
The family is in Beaumont, possibly, on the
1821 census (pdf) - Jno Marshal, 3 males and 4 females.
I don't know if you've seen this but I've put my transcriptions up as spreadsheets so you can search alphabetically -
Great Oakley here. I can't see her in my transcriptions and I can't see a Rebecca baptised in 1796 either.
The search results on FreeREG look like Wix, Thorpe and Great Oakley as being parishes where Gobeys were being baptised around that period. Of course, that might help you rule out parishes where she's not baptised and look around the contiguous parishes which haven't got to FreeREG yet (if you're using FreeREG, that is...!).