Bombay Sapphire Gin or Gordons if BS unavailable
Schweppes Tonic Water
1 Long Glass.
Take long glass and fill with ice up to 1/3rd of the height of glass.
Squeeze a slice of lime over the ice and rub remainder of slice around top edge of glass
Pour gin until 1 centimetre above level of ice
Drop in half slice of lime
Pour tonic until 1 centimetre below top of glass
Repeat and repeat again, and then again and again.