[GrahamP]

Andrew, sorry to hear of your problems.

From your original post it would seem to have suffered an over fuelling problem and some form of mechanical damage to one or more cylinders.

Have come across similar occurrences and my first reaction would be to remove all 4 injectors, as it is not unknown for the tip of the injectors to shear off due to either age or metal fatigue where they enter the cylinder.

The injector body would still be sealing so giving compression but neat fuel entering instead of fine spray giving black smoke.

If one or more tips have broken off and dropped into the cylinder would be enough to cause mechanical damage to cylinder head and piston also possible valve and seat damage.

If it is not the injectors then it will give you the opportunity to do a compression check.