In reply to afx22:
Corpse Start (V1) is good thin hands (it's the purest straight-up bouldering crack I can think of), although I'm not sure how advisable heading there at the moment is (although in fairness it can't get more banned)
Harland Globetrotter (f6A), Leggit (f6A+), Youngster's Wall (f7A). All of these are thin cracks - Harland Globetrotter has solid locks that make the whole thing much easier, and on the other two mono jams are useful.
As others have said, lapping Dog-Leg Crack (HVD 4a) is good - it's one of the few easy jamming cracks on grit that is significantly harder if you try and layback it.
String Theory (f7A) Obscure and I've not been on it, but supposedly one of the best roof cracks in the Peak.