-GB-Carpy wrote:

My work only involves small periods of time at a given chimney, so it would need to be
something I can carry up and down.

It sounds like you need to make a companion ladder to your roof ladder.
I don't know what length it would have to be. As long as the widest
chimey on your patch, plus long enough to take a crawl board.

Say your present roof ladder is made of 3 x 1 with 18" 3 x 2 rungs 12"
apart and the average chimney is 3 feet from ridge to bottom flashing.
The other needs 3 1/2 to 4 feet long plus 18 and some inches long; to
be able to put it in place and to take the connection slats.

The connecting board needs to be as wide as both ladders and the width
of the chimney.

An alternative to this short roof ladder might be a length of 3 or 4 x
1 with a block at both ends, either side of the lath. One to catch the
ridge and one to rest the crawl board in.

It's either that or build some sort of a jib. Dangerous but a quick fix
and a good way to get a disability pension.