who is Thomas 
do you mean Thomas Archer who married mary Hackett with witnesses Ann archer and Robert hackett.
I'm sorry. Yes, that's the Thomas. He was Baptized 1785 to another Thomas, and was the eldest sibling of Humphrey (and several others). Said Thomas is the one that married the Mary Hackett I had you search for. That was more of a comment to myself than to anyone. Is there a Baptism of Ann Archer c1793? All of Ann's siblings have their Baptisms recorded there except for Ann, it seems.
Mike
Edit: I'm wrong. For the Archer family I have:
Thomas 1785 - married Mary Hackett (1791)
John 1786
Samuel 1787
Isaac 1789
Humphrey 1790
?? Ann ?? 1793
?? Robert ?? 1796
Sarah 1800
I cannot locate Baptisms for Ann and Robert Archer. The family only resided in Hatton and Hoon Hay farm, both in Marston-upon-Dove...
For the Hacketts:
Robert 1785 - married Ann Archer (1793)
Mary 1791 - married Thomas Archer (1785)
Thomas 1794 - Oddly married in Birmingham to a lady whose family is from Cornwall.
James 1796
I am probably missing Hacketts, but I cannot find where they Baptised their children apparently. I know that the above Hacketts are likely the grandchildren of Sampson Hackett who married in the 1740s to Ellen Williamson and resided in Hatton. Sampson was a family name, and quite unique.