We have an artificial marble fireplace that is fixed against the wall and has a hole for the fire, we have a new electric fire which has a 60mm projection behind it meaning that I have to cut a recess into the wall to alow the fire to fit flush against the back of the fireplace, this means cutting the recess probably about 50mm deep in the inner skin of the brickwork (its on an outside wall) what is the best way of doing this as I don't want to go through to the cavity as this is full of insulation. Any suggestions would be great