Andrew
I had similar trouble in May 2011. Because of the safety/liability element, most people seem reluctant to say much. Here is a copy of my notes at that time. I hope they help you:
"Seatbelt drvr side not retracting after use. Belt is worn and shabby.
Autobarn Repco cautious with advice re aftermarket c/o unit. Too many options, what type are fitted?
Klippan website shows many models available depending on angle of pillar mount etc and a matrix helps make a choice. see several pdfs on line.
A Perth Co can service and replace belts for about $120-180 each, plus postage.
MB manual shows how to remove pillar cover and belt unit. See page Chassis 68.470
Attempted service and repair on 11.5.20, and cover came off easily with 2 screws and a downward pull. Seat slid right f'ward. Mounting bolts at pillar top and floor came off easily. One bolt/nut holding retractor assy inside frame, then out, minding vac lines to doorlock and wires for window.
Belt was frayed and twisted, but retractor reel OK. Considered replacing belt fabric but seemed too hard to retain coil spring without special tool. Noted that belt guides were back2front, and reinstalled properly. Belt now works again. Job done. Plastic bottom trim plate with 2 screws was cracked in sev places. Temp. repair w 2 part araldite reinforced with clear plastic from a bubble pack. Now replaced.
Belt assy had following marks.
Buckle - male - SB408-2
Trim plate - 116 6920222
retractor/inertia assy - SB2010; 90 90 C; 750725
The 90 90 refers to angles of installation; pillar base and top mount are vertical, ie 90 degrees; they are plumb. Also they are plumb in the other plane, ie the top is not inboard of the lower mount. The inertia lock is set for that condition. See Klippax for guide to aftermarket units."
That's it. The WA service requires the units sent to them so, if you choose that option, plan not to need the car for a while. Also, pls note there are warnings NEVER to use lubricants on an inertia reel system.
Regards
Rod.