Coal like residue on rocker cover/injectors?

Hi,

I have a 99 Vito 110cdi which started to run lumpy, smell of diesel and smoke form the engine bay. I investigated and found what can only be decribed as 'coal' all over the rocker cover, completely covering the injectors and wiring when I removed the cover between the engine and the airbox.

I have removed most of the stuff with a soft hammer and screw driver and jet wash and I think I will replace the injectors but don't want to do loads of work if I have another problem. The only other strange thing is that the airways from the airbox over the top of the engine down to the head are smeared with old oil.

Has anyone else seen this issue? Thanks Graham