Thank you for your reply (I know it is a post from a while back) no one has cracked this brickwall in the last 12 years since you posted the comment. None of the genealogy sites or +40 family trees that have Marry Fortt nee Fisk know her parents or exact details. The UK Fisk / Fiske web site has her birth year as 17-- and parents unknown.
An interesting fact that took me months to clear up is
what you said here
"It looks to me that by 1851 William has remarried?
Wm Fortt 55
Ruth Fortt 53"
William Fortt and Ruth Fisk nee James married 29 Sep 1847 in Ardleigh, Essex, England
Many people were posting on their tree that Ruth was born Fisk which was not true and caused a lot of research to unpack. Some people posted that Ruth was the mother of the Fortt Children which also wasn't true.
All William Fortt's Children were born to Marry Fortt nee Fisk
Ruth James / Fisk / Fortt had three children (one may have died young) with Thomas Samuel Fisk born About 1791 in Orsett, Essex, England and died by suicide in Oct 1843 Paglesham, Essex, England.
I feel like Thomas Samuel Fisk is likely Marry Fortt nee Fisk's cousin, brother or something although I don't know. I think more likely a cousin.
the "James Fortt 17" mentioned is my 2nd great grandfather.