1.9tdi classic gear box problem please help.

Im sorry if this has been done to death. 

I seem to have lost 5th gear. i noticed 3rd gear occasionally got a bit sticky then suddenly its not possible to put it in 5th. All the other gears work fine...Ironically 3rd is really smooth now.

Its a high mileage ex police car with full service history. Local mechanic sat in it and just said "needs new gear box" which i felt was a pretty fast diagnosis.

Any one able to advise me?

Thanks