hi,try this,on that rubber link that joins the remote linkage to the gear box.
the bolts are shouldered,and can wear,(that leaves the nut tight against the shouldr and not tight against the rubber thingy!)so even tho they are tight, when you try for reverse by twisting the shift rod,(as you move your shifter to the right)the gear box end wont move as it should,(have a look while someone trys for reverse,had this prob on my bis,could'nt get reverse at all.
take them out and replace,or fit a small washer in to take up the slack.
hope its as simple as that for you mate. :cool: