It will tend to require a new section of felt across at least two, maybe three rafters. Maybe new battens too in that area, and not just pieces.
Scaffolding, or at least a tower should be should used - as a client you should not be expecting roofers to work on a roof off a ladder - it's your fault if there is an accident.
But if they do, if the flue is up the roof it will be easier than if the flue is by the eaves where they wont have anywhere to stand.
And as always, there is the issue of whether the flue or bitumen felt (or artificial slate) contains asbestos.