Actually my understanding was that the Hope Valley stoppers were generally worked by heritage units (104s, 108s and latterly 101s after 1992) up until the Summer 2000 time table change, the 101s were the prefered class for the route. After this point there was a reduction in 101 units and they were focused on the Marple/Rose Hill workings (which they already ran anyhow), leaving 142s, 150s and 156s working the New Mills Central and Sheffield diagrams - back in the days of First North Western, I can't speak for after 2004 though.
By the early 1990s the only first generation DMUs working in the North West were 101s and 108s. The final 108s were withdrawn in 1993, following the collision at Hyde North Junction on 22 August 1990, which raised concerns about their structural integrity. http://www.railwaysarchive.co.uk/docsummary.php?docID=867
It had been intended to retain and refurbish the 108s, but following this incident the 101s were retained instead.