Transmission 'bang' whilst driving

Been out for a short run, 10miles, as I drove up slight incline there was a 'Bang😬, from the engine, grey smoke coming out from the side, and rear. The engine went into high rev's, and the car speeded up. Turned engine off, waited a few minutes and started car again, but same thing happened. Rolled car out off the road. There's a slight oil leak front offside. No fault showing on diagnostics.. Any ideas?? 

Thanks