mis-diagnosed car problem

I dropped my car into dealer (mercedes) with problem with steering/tracking. They diagnosed problem, ordered part, fixed it, returned car (had meanwhile gave me hire car which I was charged for). Problem still exists with steering - they still want to charge me!(?). They claim the work carried out was necessary as it was a problem (maybe not THEE problem) & payment must be made. They're willing to take the car back and keep looking for the original problem. Any advice?

What was/is your orignal problem? and what part did they fit and what did it fix?

If you hired a car of course you would have to pay for it, were you not told you would have to pay for it before you took the hire car. Is the Hire car company a separate company?