I have had exactly the same problem for a while, today we bought a new handle and spindle and fitted it , it made no difference at all. I couldn't find anything on the projects page on this or am I not looking in the right places. :?
I'm afraid there is nothing in the projects to cover this. Its the lock itself which contains the spring to return the handle to its original position and, as far as we know, there is nothing in the way of servicing the existing lock, that can be done to repair it. Fitting a new lock is not difficult but you may need to visit a few double glazing companies with the old one to find one which will fit.