If not under warranty I suggest you take it to a clutch clinic - sounds the most likely cause to me. Do you leave it in gear overnight and have you tried depressing the clutch whilst starting as many handbooks recommend.
I had the same problems on my Peugeot and that was a Diesel aswell. when you start the car just leave it to warm up first and go throug your gears, 1 2 3 4 5 and Reverse, after leaving it for about 5min it should be fine. Garage told me it would cost over £130 to fix, but i said il do the warmin up part first