Frontera 2.2 Auto gearbox problem?

I bought a Frontera SWB 2.2 Auto Diesel car as seen but just 500 miles later it sounds like the clutch is slipping but of course its an auto. when the car gets into 2nd gear everything works fine I have been told I need a complete new gearbox but it seems a bit of an over kill as there was no problem at all 100 miles ago!
oh we checked the oil and its fine