Use a small rectangular tin to make a neat job of filling gaps in corners where concrete panels have been fitted to a wall. In one corner of the tin cut a slot to the width of the gap to be filled. Use a sharp chisel to cut notches at each side to a depth that will take a hacksaw blade. Then saw down each side of the slot and finally use the chisel again to cut across the top and bottom.
Fill the tin with a cement filling-compound. Hold the tin firmly into the corner and use a trowel or a piece of wood cut slightly smaller in height and width than the slot to ram the filler in place. Work from the top downwards.