What is the last name of the person who was 8 months old when his parents separated?  Answer the above question based on the context below:  Johnston was born on 23 September 1994 in Dumfries, Scotland, the son of Andrew Johnston and Morag Brannock. He was given the extensive name Andrew Aaron Lewis Patrick Brannock John Grieve Michael Robert Oscar Schmidt Johnston. Johnston's parents separated when he was eight months old, and from that time he lived with his mother and three older siblings in Carlisle, Cumbria, in the north of England, where he attended Trinity School. Johnston tried out for Carlisle Cathedral Choir at the age of six at the recommendation of Kim Harris, a teacher at his primary school. He was auditioned by the choirmaster Jeremy Suter and accepted into the choir at the age of seven. Johnston's mother, who had no previous association with the cathedral, described her feelings of being overwhelmed by emotion at having her boy singing in such a "stunning building among those extraordinary voices". His mother also described Johnston's busy regimen of practice four times a week and all day Sundays, saying that it took up all of their spare time. However, she said that the cathedral staff became like a family to her son, and that "it was such a lovely, safe, close feeling for him". Johnston, who attended Trinity School, was subject to abuse and threats from bullies which drove him to contemplate quitting the choir, but he was helped through the ordeal by his choirmaster and the dean and canons of the cathedral. By the time of his participation in Britain's Got Talent, Johnston was head chorister.In September 2008, after his appearance on Britain's Got Talent but before the release of his first album, Johnston embarked on a tour of Norway with the choir, performing at Stavanger Cathedral and Utstein Abbey, among other places. The tour was conceived because the Diocese of Stavanger is connected with the Diocese of Carlisle through the Partnership for World Mission. This was Johnston's last tour with the choir. Johnston features as head chorister on one of the choir's albums, The Choral Music of F.W Wadely, released in November 2008.
Answer:
Johnston