Lower Radius Arm, Difficult to replace?

So unfortunately with all the ice and snow we've been having in the UK recently, I ended up sliding off the road on some ice in my 320 convertible and have quite considerably bent the lower radius arm on the passengers rear side, now does anyone know is it difficult to replace this? Do you need to disassemble the entire hub or is it just a question of undoing the radius arm putting the new one in and then getting the geometry all re-done?

If anyone knows the answer to this it would be most helpful, and can you get an OEM one or is it only possible to get this part directly from BMW?

anyone?

not sure what a "lower radius arm" is - that is not a common term here