As a builder I would remove the plaster 1" either side of crack. You will probably find the crack is a gap in the mortar... Remove some of the joint about 5" long break up some slate (either buy one from a builders merchents or jump in a skip) into sizes approx 1" x 4" and with a big hammer, taking care not to cut yourself on the slate, hammer the slate into the newly formed hole... Slate is incompressable and will nip up the joint... replaster!!!
Reubs
March 2010