Roof tile leak repair using first principles that consider copper corrosion, water course function

Roof tiles are sensitive to modifications and weathering - and this leads to leaks over time.
Sometimes, a roof leak is simply repaired by just replacing the offending roof tile.
What is this is not a good option?.
When is roof tile replacement not a good option?
When we found the cause of a roof leak in Sydney recently, a modified concrete roof tile was the culprit and we did not think a simple tile replacement was a good option.
So, we had to go back to first principles to repair this roof leak.....

[@sydneyshinshi]

Creative repair.