A blocked valley causing water to overflow can be cleared of debris fairly easily as a basic maintenance task. If the lead in the valley is cracked then replacing it is a bigger task one you will need to be expertly installed. Temporary repairs can sometimes be done using flash band (self adhesive flashing tape) but in the medium term the lead will need to be replaced. Well installed lead valleys can last 40 years and more.
If you have this problem give Dyson Roofing a call on 07765 257 092