Its the same on the genuine Dickson, hold in the clamp in and turn the cam while pulling on it, its quite firm so use the spanner to turn while you pull.

It should be hard for the hex to round over if you are using the correct bent spanner but I suppose it may be an imported copy that has not been hardened as well or used with an ill fitting spanner at some time.
Rotagrip do spares but not cheap, may be worth trying the guys that now make them and sell on e-bay, A & R Precision.
J
Edited By JasonB on 10/07/2013 08:01:38