1993 Golf Gti won't start.

I was driving home from work on the 10th of december 2004 when suddenly whilst still in motion the car stop and has since been unable to start. My mechanic has checked the 'brainbox' , the coil, the spark plugs and even changed the top cylinder gasket all to no avail. Please could you give opinion as to what could be the cause of the engine refusing to start.
N.B. Once the engine tried to start and we discovered a lot of carbon dicharge from the exhaust.
Please Help!!!

I assume you've checked & are getting a spark. Have you checked the fuel pressure? Have you looked at the timing marks on the cam, flywheel & distributor to be sure they are correct? Is it flooding the spark plugs?

All the things you mentioned have been checked. The fuel is spraying fine, the spark plugs are sparking and the timing has even been checked. My electrician even went as far as disconnecting the demobilizer in case it was in some way malfunctioning. I had heard that there is some sort of sensor (modulator) in the air intake line that could account for this problem. Could this actually be the problem?

You may be referring to the mass air flow meter in the intake pipe, but it usually wouldn't cause your symptoms. It would be good to get an actual fuel pressure reading to eliminate that side of the equation. Have you checked the engine speed sensor, mounted between the engine & transaxle sort of behind the front mount.