When a housebrick gets cracked (frost, subsidance and being whacked by a heavy object are all possible causes) the crack will allow water into the wall. Therefore you need to replace the brick. This is actually quite a simple task that you can do yourself.
How to cut out and replace a damaged brick.
If it is just the outer face of the brick that has flaked off, you could just remove the brick you have already got, turn it round and reinsert it. Otherwise you will need to get a new brick. Do try and match the look of your current brickwork or you will forever be seeing the obvious ‘wrong’ brick in the wall.
To remove a brick You will need to use a hammer action drill and a number 6 Masonry bit.
drill holes in mortar
Drill as many holes as possible through the mortar surrounding the brick you want to remove. Try to avoid drilling the bricks themselves. Once you have cleared most of the mortar the brick should come loose. You may need to use a hammer and chissel on any stubborn bits of mortar, but in general you find that the brick comes away fairly easily at this point. So, pull out the brick and pull any remaining mortar out. Do not push the waste mortar into the cavity (if it is a cavity wall) as this may then bridge the cavity and lead to secondary damp problems.
To put a new brick into the hole is not difficult.
Mix up some mortar. If you haven’t got any to hand and don’t expect to be doing much more of this in the near future, buy a small readimix bag of wall mortar from the diy store.
In a bucket, mix the mortar with water as per the instructions.
Now, lay a bed of mortar to the bottom and sides of the brick using a masonry trowel (A 4 to 8 inch trowels seem easiest).
Splat mortar onto the top and sides of the brick and push it into the hole (don’t push it too far – it needs to be flush with the rest of the wall but not indented). Clean up excess mortar using the side of the trowel.
Well done, you should now have a brick replaced in the wall looking as good as new.
My final tip is to wipe off excess mortar from the new brick and sourrounding area to get a good looking finish.