Today we replaced approximately 20 broken tiles on a roof and installed two colour matched whirly birds.
When replacing broken tiles, we remove the broken ones, then use tiles from the bottom row to sit where the broken ones were. We then use the new tiles, installing them in the bottom row. The reason we do this, is so the new tiles are not visible from the ground level as often they are brighter and cleaner and are more noticeable when scattered across the roof. Using the tiles from the bottom row means that they blend in well with the roof and the repairs are not noticeable and are aesthetically pleasing from the ground level!
You will see what I mean when looking at the photos :)
We also installed two colour matched whirly birds which will ventilate heat from the roof cavity, making the house cooler in summer and also allowing trapped damp air to escape.