[Moppie]

Its not a clutch problem, which is what the release bearing is part of.

Its clearly either a problem with the external linkage, which soul be easy to find by getting under the car and having a look around.
Or, its an internal problem with the shift mechanism in the gear box.

If its the former its just a matter of finding it and fixing it, any competent mechanic should be able to. Infact any compenetent mechanic would have found it was a problem with the linkage/shift mechanism pretty quickly.

If its internal then IMO it would be cheaper and easier just to replace the whole box.
Often you can fix the problem, but find out a week later its caused other problems else where in the box. i.e. worn or bent shifter fork, worn syncros, etc.