When removing a brick from a cavity wall, always take care that chippings do not fall down into the cavity. If a large piece of brick jams across the cavity it will form a bridge between the outer and inner walls along which dampness can travel.
Reduce the risk of this occurring by using a bolster chisel and a club hammer to cut a V down the centre of the damaged brick to a depth of about 75mm 3in. Hammer the bolster into the mortar on each side of the brick, angling the bolster towards the ‘V to help loosen the brick. Take care not to chip adjacent bricks. Then hammer directly in the centre of the V until the brick splits in half. Work from each side of the brick in turn to prise it out.