High Exhaust Back Pressure

[mibo]

So two garages have told me they dont think its blocked and dont want to remove it. Ive tried removing it, but I cannot get to the front collar from the engine bay or underside of the car, so I'm flushing it through from the DPF sensor and out the back end which I have removed.

Its definitely full of something. Looks like a combination of soot, ash and some oil.