Hi Monica,
Many thanks for your input.
I did not see the Sally Ann Mahood reference until you pointed it out. She married Edward Johnston on 26/6/1860 @ Stoneykirk, aged 46.
Regarding her son, John Diamond, I have him in the 1841 census living with John, Elisabeth and Sarah Mahood in Stoneykirk.
Strangely, I cannot find any reference to them in the 1851 census?? Will keep searching.
On John D's & Mary Lavery's wedding cert. ( 29/4/1862 ) both father and son are stated as ploughmen. Mother - Sarah Mahood.
On John D's & Agnes Gray's wedding cert. ( 2/3/1885 ) John D is a mason.
Parents - William D. - farm labourer and Sarah 'Hood' - Housekeeper.
Also of interest or coincidence is that the male witness is a William D Hood.
The plot thickens.
John D's death cert. 21/3/1921 - profession - Mason. Lavery & Gray wives stated. Age- 77? [ 82 actual] Informant - William Campbell; son-in-law - was 2nd husband of Annie Bella Diamond.
Parents - John Diamond, mason, deceased, & Elizabeth Diamond nee Caldwell, deceased?

?
Was John farmed out to one of his Father's brothers thus the assumption they were his real parents?
Did William Campbell just make it up to fill the box?
This is what keeps me going - there is an explanation, I just cannot see it yet.
Regards,
Colin