I can't see a marriage for a Henry Heat either, but there is this one in Withyham worth considering maybe Henry's father or a relation
Henry HAIT married Ann Ovenden 19 October 1752
BURIALS
Anne HEATH widow buried 25 June 1810 Withyham
Henry HEATH buried 6 June 1778 Withyham aged 62
Could not Henry and Ann, married 1752, be the parents of Sarah born 1773? Thus eliminating the need for another Henry in between?
They marry in Withyham, toddle off to Cowden for a while. After the second(?) Henry is born (or baptized!) in Cowden they go back to Withyham.
On the other hand, a Henry Heat/Heath is in the Withyham land tax records from 1764 (but not before?). In the lists he is after William Ovenden of Blackham Court.
https://www.familysearch.org/ark:/61903/3:1:3Q9M-CSJ8-BWXN?cat=340725
I'm not sure Henry Heat and Ann Ovenden (who married in 1752) were the parents of Sarah Heat (christened 1773). It looks like Henry was born in 1716 in Hawkhurst (to Richard and Ann Heat[h]) whilst Ann Ovenden was born in 1735 in Withyam (to William and Ann). If they were the parents of Sarah then he would be 57 when Sarah was born and Ann would have been 38.
It is possible I suppose however I think the confusing thing is the fact there are two Henrys born to parents named “Henry and Ann”
Henry christened in Cowden on 17 April 1753
Henry christened in Cowden on 22 March 1767
If the first Henry died then the second Henry could be another son of Henry Heat[h] and Ann Ovenden, and in that case he would be too young to be Sarah's father. This in turn would mean that Henry Heat[h] and Ann Ovenden wouldn't be her grandparents.
I suspect that the first Henry (born 1753) is Sarah's father, unless as you say she is the daughter of his father (ie Henry Heat and Ann Ovenden married in 1752). It's just confusing having two sets of parents called Henry and Ann – unless they are the same people.
The mention of Henry Heat on the tax list after William Ovenden is very interesting and makes me think there is a link.
It's a bit of a tangled puzzle to be honest.
Time to go back to my notes I think and see what I can work out.