Sorry for the outrageous resurrection.black wire earthed to gearbox
Yeah I think it is quite a thick one. There's metal showing and it looks pretty rough!If its a thick black wire, yes it may be causing your problem, its the negative terminal to supply the starter motor.
The small wire should have 12v approx when you turn your key to the crank position.
Its not a problem if metal is showing as long as its no where near a live cable as the whole car is connected to the negative side of the battery. If its broken or damaged it could be a problem.
Get a jump lead and copy the connection, starter body to gearbox or battery negative and try cranking it.
Ian.