It's not clear from this photo that the chain on the right (of the photo) is slacker than the other. Despite very careful setting up of the axles and even more thoroughly checking that the chains were actually the same length, one chain insists on drooping. At present, I don't know the cause of this although I expect they will equalise with running in.
I also previously referred to items called chain 'slippers' and these are in the photo below.
Whether these resemble Sentinel's idea of a chain slipper, I don't know; however, they did the job required of them.