It is a steel upright, the magnet confirmed that!
Part numbers are an issue here. They appear to be few and far between. The actual arm I need (the rear lower) has no part numbers on it, despite having everything else stamped on it, including an Audi logo. I assume they used the same arm with different bushes for different applications, so the number isn't stamped on it, if it's just been printed instead it's long gone.
The front lower part number 4D0407155N (stamped on side) from the nearside front arm, doesn't seem to be the actual part number I need. I'm assuming again that it's for the arm and not the complete assembly including bushes.
Adrian I might take you up on those bolts, I may also need to strip off and refurb / replace one of these callipers! The bolts would be handy
