I have a self sealing Disco too, similar in nature to yours.
The centre spring on the gear stick went. We lived with it, only occasionally getting reverse when it was actually 1st that was required. Then it became damn near impossible to get reverse. I even asked on MC how to fix it.
Then, all of a sudden, everything was back to normal. Nice centre spring, easy reverse.
If only I could have a self healing "clunk" and self healing rust. :D