By ball joint, I assume you mean track rod end? Easy peasy (once I spoke to Chris - welsh mod/mot dude ) job that took me over an hour to do the first as I couldn't get the bastid thing undone (then realised I was trying to unscrew it the wrong way ) and less than 15 mins to do the second as I know how it was done.
Why do you need to replace the steering rod arm
And I want more shiny blue arms under my car!! Just 2 pairs of rear ones to go - got the rest