Ford Fiesta 2013 Ecoboost Manual Gearbox issues

Hi All

Really looking for some help here as all I seem to be doing is throwing lots of money into a car that still isnt working properly.

6 months ago i was having gearbox issues, first I had the clutch plate replaced (also did the cam belt), then a week after the gearbox was replaced. Knowing that there are issues with this gearbox my mechanic was able to get a reconditioned one with a modification to resolve the issue with the syncros. Gearbox was great for about 6 week before I started having issues with getting the car into first gear, only when down shifting. I took the car into the garage and they gave the car back and said until it happens all the time they cant diagnose it. About 6 weeks after this it finally happened on the m25 in traffic it refused to go into first gear. The only way now to get it into first gear now it that I have to stop the car, put it into neutral then back into first, if the car is rolling it will not go into first, every other gear is fine. The garage has taken the gearbox out apparently and said that there is nothing wrong with the gearbox, its most likely the gear linkage and have stated that I have most likely stretched it because I have not been engaging the clutch properly.  They have stated that this is additional costs even though the gearbox part is under warranty for 2 years. Can someone please give some advice with what to do as this garage has probably had my car for 9/10 weeks now in the last 6 months.