This is a longshot.
There is a John Howat - Ag. Lab., wife Jane (Jean) and four children, Samuel 11, Thomas, 4 Jean 6, and Marrion l, living in Kirkcaldy, Durrisdeer, Dumfries on the 1841 census.
On IGI is listed a Henry Howatt and Janet Gilchrist, married on Nov. 13, 1802 in Bonhiel, Dunbarton. They have children listed as follows:
James 1803
John Feb. 5, 1805 ***born Bonhiel, Dunbarton
Janet 1807
Portfield 1808
William 1810
Henry 1815
Marrion 1818****
The only reason I thought perhaps this might be the family you are seeking is because your John Howat named one of his children, Marrion. Perhaps this is just coincidence, but perhaps worth looking into.
Torrygirl