This may not be your Alexander Craig and Janet Kerr, but this MI may be of interest.
The inscription is from the United Presbyterian Graveyard in Stranraer.
The stone would seem to be badly weathered -- the inscription has gaps in it.
Erected [----------] Alexander CRAIG, Farmer in Bine, Kirkcolm
In Memory of
Agnes KERR his wife, [---] died Nov [3d] [1]826 ag[ed] 3[2] years
[--------]oth[--] Margt McCULLY, who died July 29th 1834 aged 69 years
Also Peter CRAIG his son, who died [-----------] 1869 aged 23 years
Also the erector of this stone, Alexander CRAIG, who died 11th May 1[87-] aged [--] years
Also his wife Jane McCLURE, who died 187[-] aged [--] years
[-------------] Margaret [---------ter] [----------] wife [---] Stratt-n [-----] London, [----] who died [-----------] aged [------]
John CRAIG, son, died [--] July 18[-]4 aged 7[3] years
Sy
Had to revise this -- got the graveyard wrong first time !