In reply to richard.hyde: another vote for north Lees, its about 15min walk to the base of the crag, okay unless you're bringing a 30 person tent.
with ref to wild camping the rangers do drive round at time late and early morn and they will move you on if you're found, the camp site is nice and cheap and has all the amenities you will need and its a 20-25 min walk to Hathersage across the fields.
or if you guys have wheels and dont mind a short drive you have Hardhurst farm near Hope and Castleton, just behind the travellers rest pub, again quite cheap lots of space, trees to chuck up slacklines etc.