I currently own a 2005 ford mondeo, 137K on the clock and used as a taxi. Ive only done 4K on the car since i bought it 6 weeks ago.

On thursday night i worked and the car was running perfect but started the car up friday morning and i drove 100 yards and the engine died. 

Now when you start the car up, there is a horrendous constant banging as if something is spinning and catching something and this noise is coming from the long black box behind the injectors. 

Once the engine is running, if you put anything on the throttle, the car dies again. Also when engine is running a tony bit white smoke is coming out of exhaust that smells like diesel. Any suggestions? I have a video of this

A video would help, as for banging is it like a loud knocking noise rather than bang?

A knocking noise from the engine is never a good sign, without hearing it it's hard to diagnose, but a knocking engine is most commonly when the bottom end knocks and is a full rebuild or swap the engine.

At 137K miles the injectors are also probably about knackered if they are original, so it could also possibly be injector knock. That could well explain the diesel smell. But injectors are not a cheap fix either usually, not sure if that particular engine requires new injectors coding in like the Focus 1.6TDCI engine does.