Clear the route first
Moss, leaves and pieces of mortar can hold water against the edge of the valley. Remove debris carefully from safe access before deciding the lining itself has failed.
The top of the valley matters
A leak near the lower end can start much higher up. Look where the valley meets a ridge, dormer or adjoining slope. Rain entering there can run along the channel before finding a weak point into the roof.
Lead and preformed valleys are not repaired the same way
Lead needs allowance for movement. Preformed channels rely on correct support and overlap. Mortared edges can crack or become detached. The repair should match the construction rather than cover everything in one coating.
Access can dominate the price
A valley may sit in the middle of the roof, beyond safe ladder reach. The material needed for the repair can be cheap while scaffold or other access costs more. Ask for access to be included clearly in any quote.