Blowning Exhaust - Sensor?

I have just purchased a Ford Fiesta Flight 1.3 3 dr on a Y reg. The engine management light has come on when the engine had warmed up (1-2 miles) and the car started to run rough. The car sounds like it's the exhaust blowing and on the MOT (may) there is a cautionary for a gasket. If it is a gasket on the exhaust, would this cause the engine management light to come on and loss of power when accelerating. I don't think it's the alambda sensor/s as I've had those problems before with fords - any help or advice please as I may need to take the car back and get a refund.