I've done it many times, when I used to live up north.
It's a beautiful and dramatic scenery, a narrow gorge with this steep climb in the middle.
It's just over a mile, at a rather constant and painful 20% gradient. If you look at the profile on mapping softwares, it looks like it eases in some sections, but I've always found that from cattle grid to cattle grid it's pretty much relentless.
I've done it with 34 x 23, but I suggest a 25 or 27 for more comfort.
The other side, what is known at the Barber Booth, from Edale is also a very nice climb, a touch less steep (15-18%) but a bit longer.
As described on my website http://ridewithugo.xtreemhost.com/mamtor.html