The 1907 25in OS looks as though there are a total of three wheels in series at this point. The second wheel is just visible under the launder, while the third is somewhat lower, presumably fed from the second wheel discharge.
I guess the upper wheel is a mid backshot, the middle wheel appears fed from the top while the lower is difficult to discern!
A fascinating place
WWW.britainfromabove.org.uk/image/EPW045663
Shows the drumhouse and mill atthe top of the incline - if you register (free) you can zoom in considerably!
There are several photos of Delabole in this collection.
Grahami
The map is the territory - especially in chain scale.