You need to identify exactly which part is worn, has it failed the MOT or something?
There are more ball joints to the rear of the foremost arm which may also 'knock' when worn but they are more difficult to identify as worn unless you partly dismantle the suspension.
Also don't forget the track rod ends too.
I personally would be looking at the two joints at the end of the front and rear arms. http://cgi.ebay.co.uk/Mercedes-W220...39:1|66:2|65:12|240:1318&_trksid=p3286.c0.m14
This is the foremost complete with ball joint.
The rear ones are quite expensive but you can replace the inner ball joint on them. If the outer one is gone, you need to buy the complete assembly.
Hope this helps.