Thanks for the input guys, appreciate it.

I didn't have any products lying around which are specifically designed for removing rust, nor did i fancy spending hours polishing the thing with steel wool so i improvised...

...the rusty parts are now sitting in some Pepsi Max! Apparently carbonate (fizzy) drinks remove rust, so i though i'd give it a go. I'll leave them in till tomorrow and see if it makes any difference. I already took the bearings apart and blasted them with nitro cleaner, they're spinning much more freely now. I also chucked them in the Pepsi to keep the ring gear company.

If it works i'll just replace the worn plastic diff case, and make sure the thing is watertight. Oh, and i'll try and make some money selling "iron rich" Pepsi, because *it's good for you*