Just thought I'd wrap up this thread in case anyone else has the same problem.
When I got the sliding door runner off it looked pretty bad.
I had to use an angle grinder to clean it all up!
I treated it and (since it is out of sight) just used metal paint to finish it off.
You need to take the sliding door off to do this job. It takes two people to control the weight so you don't scratch anything. You take the long metal runner panel off first. You can pull this off once you have the two screws out. (if you have a camper this saves taking the inside out). Make sure your fingers span the clips when you pull so you don't damage it. The clips should let go or break. You have to fiddle panel out with the door three quarters closed. Best to use help and cloth to avoid any scratches.
The clips are available from VW or ebay. See
this thread.
To remove the door-
Support the half open door (on blocks or a jack) and remove the two splined bolts on the bottom roller.
Lower the front of the door slightly so the top roller slips out.
Slide the door fully open so the roller slides round the curved section of the runner and off the end. You will need to help it but it goes. No need to unbolt any rollers except the lower one.
I sealed the black plastic cover with silicone. (Then started on the second sliding door!)
Thanks for your help guys
