while I had the car in bits I removed the DPF and gave it a good clean with a pressure washer and Mr Muscle oven cleaner.

And what makes you think that will clean a DPF properly ?

Think of a DPF as being something like a book, 1000 pages thick. All the pages have lots of holes in, which, over time, get clogged up with particles. When most of them are blocked, the DPF is clogged up.

DPFs can be cleaned, but it involves blasting VERY high temperature air through them for several hours.

I'd suggest that your method might just clean the outer layer of the DPF, but is going to do nothing for the innards. In other words, you've cleaned the first and last pages, and done nothing to the 998 in between.

That just the pressure differential measured across the DPF,

Not the exhaust pipe pressure itself,

could be it's blocked all the way back,

'course the easy test for that is remove the exhaust and test run it.

But I'm curious,

What's PP live data for the rail pressure when all this is happening?

Black smoke "could" be air related or could be excess fuel rail pressure which would account not just for black smoke which would be in efect "flooding" the cylinders which would then flood the DPF which would then clogg up which would then............

You can see where I'm going with this...........

back to where you started.

Idle speed rail pressure is around 200 bar if I remeber right.

What's yours reading?

These are notorious for DPF issues, however........Excessive black smoke?

I would do an injector leak off test, sounds like one of your injectors may be knackered, if this is the case then 2 things will happen, the rail pressure will drop causing the vehicle to go into a limp mode, and secondly the excessive black smoke will just block the DPF again.

As much as DPF's are a PITA there is usually a reason why they block up.