Hello Jon
William son of Thomas Headland was baptised 13 Dec 1719 at Souldrop, this does not name mother as Dorothy although all subsequent childrens' baptisms do. This William, son of Thomas & Dorothy Headland was buried at Souldrop on 30 Dec 1720.
Mark Partridge married Elizabeth HEDLAND on 5 Feb 1746-47 at Sharnbrook. Subsequently as a widower, Mark Partridge married Elizabeth Smith at Sharnbrook on 7 May 1756. Brother assumed? Richard Partridge was a witness. I've not found a death/burial of Elizabeth Partridge prior to that second marriage. All children baptised to Mark & Elizabeth Partridge are dated after 1756.
I've taken it that Mark's first wife was Elizabeth, daughter of Thomas & Dorothy Headland baptised Souldrop on 6 Nov 1726 - on the basis that there are no other likely candidates. The Elizabeth Headland daughter of John & Ann, baptised at Sharnbrook on 30 Sept 1715 was promptly buried there on 4 Oct 1715. Now that there's a marriage of Elizabeth HADLAND to Joseph Goodman at Souldrop on 16 Sept 1750 leads me to question the Mark Partridge marriage. However I have no proof either way as to who she was?
You say you descend from Thomas & Dorothy, via Thomas Hedland & Hannah Curry who married at Melchbourne on 6 Oct 1755 where the PR says he was from Newton Bromswold. Have you any evidence that he is the Thomas baptised on 9 Feb 1729 at Souldrop.
Cheers John