Tom
Is it the top or bottom ball joints that have failed?
I've had a knock for several months that gradually got worse until yesterday morning I had a look and found the top ball joints were very badly worn. Easy to check, jack and remove wheel then squeeze the ball joint with your hand. There was about 2mm play in my nearside joint!
Replaced them in the afternoon with genuine Merc parts at £105 pr, took about 15 minutes each side to fit, very straight forward if you have a ball joint splitter and Torx bits, and it has transformed the car!
Steering is now much smoother and quieter and (nearly) all the knocks have disappeared. According to the parts chap it is a common problem.
If it's the bottom joints then I am told they have to be pressed out of the steering knuckle and so much more work.
Hope it helps
Phil