Thanks for the clarification portland_bill - i quite agree that the car being broken into is highly unlikely using any method, unless someone wanted to steal the raisin encrusted child seats out of the back, in which case i'd kind of feel sorry for them.
I did actually try rubbing a pencil on the key to get some graphite on there (it worked great on my front door key a few years ago), but yes you're right it needs taking out and examining properly, with a good clean while i'm in there.
I am also considering changing it to remote locking if cleaning doesn't work and it all needs replacing anyway.