Replacing soakers between former and roof

Remember that soakers are right angled so they continue up the cheeks of the dormers as well.

Think soaker, slate, soaker, slate then dressed around the very tops and the very bottom. Essentially everything overlaps each other to give you your waterproofing. All they are really doing is waterproofing the right angle join where dormer meets roof.

Some dormers have full length valleys rather than soakers. Essentially a long gulley. But really they do the same job.

So the job isn't as easy as stripping off the tiles/slates and away you go. They may have to strip off whatever material has been used to clad the sides of the dormer as well to be able to remove the old soakers depending on its construction. Some soakers sit outside edge like a brick chimeny would have, some soakers and flashing are put on before cladding etc, not enough detail on your post to assume.

Zinc is much more expensive that felt soakers.

Id have a look at lead replacement flashing and price up the materials for the job, you can then decide whether the residual is a fair labour price. Make sure you keep the old zinc and scrap it yourself.

Id say your shopping list would be

Potentially some replacement tiles or slates, stripping back can lead to breakages

Valleys or new soakers

Adhesives, nails etc depending on manufacturer fitting requirements

Material to replace anything broken, damaged on the cheeks if applicable