Hiya
It really depends on the size of the whole / crack and the condiction of the rest of the valley. If the valley is in good condiction and its just a small crack, then the roofer will just paint some aqua + over the affected area.
If the area to be painted is too large or too far gone, they might just cut that area out and patch a new bit of lead in, with 6 inches cover both ends.
If the whole valley is gone, they will replace the whole thing
Really depends on the condiction of the valley and your insurance company
CCK
Slater/Roofer