Fireplaces, Heating Stoves, Flues and Chimneys - How to knock out fireplace?

I want to widen a fireplace in brick-built chimney brest to take a woodburner. Existing dimensions are brest 147 wide, fireplace 43 wide 37 deep, concrete lintel 120 wide (centimeters). The lintel is wide, but it is only one brick-width deep so only supports the outermost layer of bricks. I need to remove a brick width from each side for the full depth of the aperture. I am happy to remove bricks in the outer course (beneath lintel) either side of existing aperture, but how do I remove the further 2 courses of bricks to widen the aperture across its full depth? Thanx.