Problem with 1st gear?

Have got transit tourneo mini bus 52 plate.Slave cylinder went recently and mechanic fitted new one plus dual mass flywheel and clutch at cost of 570 quid!
Now bus keeps jumping out of 1st gear.Ok with all other gears though.Mechanic tells me it needs new gearbox or repairing by gearbox specialist.Everything was fine until clutch etc was fitted so how come this has happened?Any ideas/advice on what's wrong and best thing to do next would be much appreciated.

Pete reid
February 2009