The first thing I would try would be to burn the DPF out, I would do this by using my local motorway network to do a 50 mile run and use manual mode to do it in a lower gear to keep the revs between the 2.5/3K range so it won't be too hard on things but will get the DPF hot and keep it hot for long enough to burn the carbon out of it. Best done at night as the roads should be free flowing.
See if it helps then go from there.
See if it helps then go from there.