Very good question! I took the derailleur apart (it was rusted and very, very dirty) but struggled a bit to put it back together - think I've managed it but now just need to get around to putting it back on and lengthening the chain again.

And yes, I'm a bit of a magpie in terms of shiny things, plus I'm a bit paranoid about rust - seeing as how this is a) my second ever bike and b) it's older than I am. However from everyone's comments it seems like that this is nothing to worry about - as long as it's treated. Like I said, I'm desperate to get it out on the road so I can ride it properly, but I want to make sure it's in good nick first.