I am having my chimney stack repaired as lead flashing is leaking

To obtain a professional finish that will last years you would need scaffolding. As a tradesmen it is difficult to work off a ladder to do such a detailed job properly especially working with lead and slate. As a homeowner you are also the principal contractor as you appointed the work so have shared liability for health & safety. Your decisions regarding the projects cost , time, or resources can affect health & safety, potentially making you liable.

Replacement lead work around a chimney stack to a skate finished roof shall require a safety scaffold … either fixed or mobile … in essence a safe working platform . Should the pitch of the roof be fairly low and the height of the roof in question be also low , a roof ladder could be considered for the works and would be quite acceptable to be within Health & Safety guidelines if acceptable by the forgoing Contractor/ tradesman agreeing to the spec and works .

Replacing lead flashing on a chimney with a slate roof will require professionalism as you will need scaffolding to do the work properly to avoid any damage to slates as they are very delicate easy to break not like a normal tile you can generally stand on a tile or move it to sand on battens

Unfortunately this work will require scaffolding to safely access the area.