Like fuck will they have to strip the gearbox. It's almost certainly a linkage issue.

Your first mistake was taking it to a main dealer.

Does it do it all the time, or just when it's cold?

I'd get them to check the linkage as long as they agree to do it for free - you drove the car in there fine, and now it's not. Failing that I'd take it to a reliable independent who will fix rather than just replace.

In the mean time pop the gearlever gaiter off (squeeze the sides and it should pop out easily) and have a look while you're changing gears - make sure the white plastic bits are well greased up and just check that it all looks as it should. It is possible to adjust the linkage at the gearbox end, and it's easy to do it too far and 'loose' a couple of gears, so I wouldn't recommend doing it yourself.