Thanks for the info chaps.

I decided to remove the DPF and physically have a look at it before shelling out on a new one. Didnt look too bad, blew it out with an airline then washed it out and refitted it.

Rechecked the exhaust backpressure and now i have 7.1mBar at idle and 45.7 at 200rpm - a dramatic improvement and well within the specs! Test drive just completed and it goes like a "scalded cat" again so looks like it was the DPF clogged up affecting backpressure on the engine.

The tuning box is disconnected at the moment so looks like we need to revisit the settings on that (customer has it turned up too high i bet) to prevent excessive soot/smoke from clogging the DPF again.