It disappeared from the menu for a while but it was definitely there when I lived in Edinburgh in 2014 and 2015. My version as noted watching the bartenders is:
50ml Canadian whisky
1 tsp maple syrup
10 ml freshly squeezed grapefruit juice
1 dash Angostura bitters
1 dash grapefruit bitters
Garnish with a twist of grapefruit zest
50ml Canadian whisky
1 tsp maple syrup
10 ml freshly squeezed grapefruit juice
1 dash Angostura bitters
1 dash grapefruit bitters
Garnish with a twist of grapefruit zest