Hi Tom
If it is rusting up in there it is quite easy to take the barel out of the door (have a look at a haynes manual).
Once you have done that you will see A circlip holds a cam on, pop that off and then remove the spring (make a note of how it goes!).
You will then have a clear view of all the tumblers and be able to put a lubricant in there and get the thing working smooth again.
I snapped a key in mine and had to take the barrel out to remove it, once I lubed it up it now works better than ever!