Buist was born to Lieutenant-Colonel Thomas Powrie Buist RAMC, an Army doctor, and Christian Mary Buist, both from
Dundee. In a 2008 interview with Malcolm McBain, Buist described his grandfather,
Robert Cochrane Buist, as "a brilliant mathematician, physician and obstetrician, but he subscribed to every rebel cause that he could. He was an early socialist, an atheist, a vegetarian, a eugenicist, and a promoter of Dundee's now heavily-used crematorium". After his retirement, Buist wrote letters to
The Guardian criticising the Church of England's attitudes to same-sex civil partnerships and defending
Peter Mandelson against homophobic attacks in the press following the events that led to Mandelson's resignation in January 2001. ==References==