In reply to gjw5670:
Melvin Bragg is the final exam really to bouldering cracks in the Peak (you could always head down south to All Elements if you were feeling really keen...)
Others in rising order of difficulty that I can recommend:
Definitive 5.11, Burbage
The Kursk V5, Burbage
Butterfly Crack V5, Warncliffe
Picalli's Pickle, V6, Robin Hood's Stride
Savage Me Softly V6, Stanage
Boffwidth the Silly Way V6/7, Rivelin
Bob Marley Extension V8/9, Stanage
Send me an email if you want any particular beta or info for other destinations!
Tom