Rover reverse gear

My son's Rover 414 1990 has lost reverse gear after having the clutch replaced. The garage who replaced the clutch has told him it is the lower linkage, but I have replaced this and the movement of the selector arm on top of the gearbox looks OK. Does anyone know of a common problem with a linkage on this car which would be easy to solve or are we looking at replacing the gearbox? It is worth doing as although the car is 14 years old the bodywork is immaculate and it has only done 60,000 miles. BB