I've had a look on twinturbo.net and it looks like a new lower arm includes the ball joint, which can be succesfully pressed in/out (if you can find a replacement), but there is a risk of damaging the arm in the process as it's apparently a bit flimsy.
The ball seat is a separate cone shaped bit of metal which just sits on top of the ball joint, but I don't think that replacing it is going to do much good (which is a shame as it looks cheap).
Does anyone know how much a lower arm costs and has anyone been able to source and fit a ball joint separately?