It all comes back to the one issue, the car is not suitable for short distance driving at low revs for a sustained period of time.
Once you have managed to clear out the DPF then run it at high revs for longer in lower gears and it should be fine
Better quality fuel may help but not entirely convinced
Another issue that can occur from slow / low rev driving is that the turbo variable vanes can gum up with carbon deposits and although this can be cleaned with treatments that can be found on eBay it will occur every so often unless the carbon is not allowed to build up again