Looking at those chimneys,I'd be more inclined to cut them down, and then rebuild just the bottom sections, or take them out completely if they're not being used. The bottom of the chimneys look in a serious condition, and I'm not sure if repointing them is sufficient, and will need redoing again in a few years or so.
The scaffolders will lay a board in the valley, and then other boards up the slope of the roof, and then construct a collar around the chimneys. But be very specific as to what part of the chimney you want to access, as they have a bad habit of assuming you just want to work on the chimney pots,and then bugger you for access to the rest.