A common cause of a leaky roof can begin with the valleys. Old rusty valley irons will start to leak as they rust out.
There could also be the common problem of an over bedded valley. Usually the best option is to replace the whole valley with new zinc or colourbond one. Then depending on the owners request the valley can then be finished off with correct bedding and pointing or left dry (no cement work) with the option of adding a tar based material called valley seal.
The tiles are replaced and the valley is bedded and pointed