Hi Peter

It is with considering why the plastic pipe melted?

The metalic section of pipe is there to withstand the heat generated at the DPF & it is sufficiently long enough to protect the rubber/plastic section of hose that connects to the differential pressure sensor. hot gasses should not really travel that far up the metal pipe & I wonder if you might have a small leak nearer to the pressure sensor.

I had a 307 hdi with just this fault a while back. The hose was burned just like yours & I later found a tiny crack in the rubber hose at the sensor, that allowed a flow of hot gas to flow through the metal pipe and cause the damage.

These differential pressure sensors are blind, (like a blind bolt hole) & there should be no passage of gas through the pipework, only pressure variations.

A crap explanation but I hope it makes some sense!