Margaret Burness is known to be William's wife as this fact is mentioned in "Aberdeen University Review p18 Volume V 1917-18". The significant date is 14th August 1752 and Margaret had two children by then.
Birth/Baptism record Margaret BIRNESS(Old Parish Registers Births 258/ 20 23 Fetteresso ) 7th October 1725 [1] Birth
Marriage would have been prior to birth of first child, Jean, born 19th May 1750.
Death date is likely to be on or after 11th October 1764 when William's daughter Ann was born and before 8th August 1768 when William married Anne Dickson.