It's could be that there is expanded metal covering gaps, or used as a "key" to get the render to stay on. The lintels are quite likely to be concrete. Can you supply a picture of the outside of the house, taking in the brickwork above and around the window? Your best option is likely to be to hack off some of the render around the crack - don't go at it like a maniac, else you may take off more than you want. Paint the surface where you removed some render with PVA 1:5 PVA:water, apply new sand/cement, plaster, or even pollyfiller mixed with sand. Leave slightly below the finished level. If there is expanded metal, make sure that you press your filler well into/through it. When it's dry, a day or two, finish with polyfiller.
What tends to happen is that if there is any movement, sand trickles down behind loose render, and the gradual wedging effect makes the cracks worse, and pushes the render/plaster outwards as well.
Fill the cracks around the lintels, or, better, patch the render.
Now, the crack around the wall is perplexing (as is the colour scheme!). Is the ceiling to wall joint OK? Have any works been done to this house? Has it been re-roofed? I notice in your picture there's a step in height for the crack as it approaches the corner. Can you supply any more photos. of this crack, and the corresponding outside?