Just a quick update. I finally got around to having my trip on the Hope Valley yesterday. A Mersey seated 142 turned up on the 11.14 out of Sheffield. Luckily I'm not too tall and can cope with the individual low backed seating. The other two diagrams were a 150/2 and a 156.
Now from looking at the timetable, the 15.45 return from Piccadilly should have been the 142, but the 150/2 turned up. Have I confused something here, or do the units somehow swap over at some point? In the end I settled for the 150/2 for my journey back to Grindleford.