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.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!