I'm guessing this is something not good??

As in, this is something that could damage the engine if some of that broke off and went it?

its not mud

its a mix of oil and exhaust gases!

normal but rather nasty

That's the result of exhaust gas and oily muck from the crank case breather meeting. It will probably benefit from a clean, I've been meaning to do mine for quite a while.

That's the throttle valve? Crikey, and I thought mine was bad.

nice....

That's actually quite clean compared to mine.

any guides on cleaning this? guessing mine will need doing..., would it be the same as a petrol engine? a bit of carb cleaner and everythings fine

This is the level of oil you get in there with an EGR delete!

I expect it is still like this now.

Mine wasn't quite as muddy when I took it off. More.... sooty

I put it down to running a crank case breather so not getting the oily air sent back in to mix with the soot.

PS: I think soaking for a day or two in a pot of diesel cleans them up nicely.

It's hard to believe manufacturers have to design cars to stick so much muck back into the engine to keep emissions down. One decent lump of that carp hitting a valve seat as the valve closes and you have either a sticking valve, or hot spot in the valve seat.