It's most likely to be the gearbox extension that houses the gearlever. It is mounted on the gearbox casing with four bolts with neoprene or similar bushes. They are known to deteriorate and allow the extension to droop or drop messing up the lever to selector alignment. In more extreme cases, the estension can foul the propshaft - you'certainly hear it.
New bushes are obtainable from Rimmer Bros. Replacement involves removal of the seats, gearbox tunnel cover trim and the cover. After removing these, access is easy and you can check the condition of the gearbox rear flexible mounting, handbrake and cable and check and grease the UJ and the sliding part of the propshaft. Probably a day's work if you're fast, two days if you're paced and want to clean up a hidden part of the car.
Totally agree, those rubber bushes are prone and replacing them cures a myriad of selection problems. For sure the first things to try, after that bleed the clutch and suspect a weepy slave cylinder before you start stripping the gearbox. It's a very good gearbox really and 90% of the problems are the bushes. Good luck with it.
