The original part (447 857 511 01C) is NLA from VW, and at least where I am, its hard to find a used one - some of the used ones have exactly the same problem as mine.
I found a very close replacement part from a Cabriolet that is more easily available (new from VW Heritage, and on eBay): 811 857 511 01C. This mirror uses the exact same mounting arm, so it engages the plate on the screen with the allen screw and it locates the mirror in the same place as the original. The mirror itself is almost identical in shape and size - not quite - but very close. I installed the replacement today and can once again drive without fear of a mirror flying around the interior of my car
Update: It seems Mk2 Sciroccos have two different types of rear view mirrors, both of which mount to the windscreen. My MY1984 has a mount about 3" from the top of the windscreen. The arm drops the mirror lower, and has the ball socket on the rear of the mirror. Other Mk2 Sciroccos have the windscreen mount lower - about 6" from the top of the windscreen, and the arm is pretty much level. The mirrors are different and not inter-changeable.
Another update: The Mk3 Golf mirror can also be good, inexpensive alternative for the cars with the ball socket on the rear of the mirror and the higher mount (see more details in the post below).
