What lead is doing
Lead is there to bridge a junction and move water back onto the roof covering. Around chimneys and walls it may be stepped into mortar joints. On other details it may form a tray, gutter or apron.
Splits and lifting edges
Large sheets can split if they have not been allowed to expand and contract. Smaller pieces can work loose if wedges or fixings fail. Wind can then get beneath an edge and the first sign indoors may be a stain some distance away.
Mortar is not a substitute for the detail
Mortar fillets are sometimes used where proper leadwork would be better. They can crack away from the wall or roof covering. If the joint has already been repaired several times, simply adding more mortar usually makes a bulky detail without dealing with movement.
Ask what is being replaced
“Repair the flashing” can mean anything from dressing one lifted edge to renewing a full run. A quote should make clear which section is being worked on and whether adjoining soakers or pointing are included.