I always spray Dr Magic/Oven Pride into them on my TDI, it foams up and drips out black. Leave it overnight and then hose them next time you clean the car - repeat 2-3 times. I do this annually before the MOT so they think the DPF has no 'issues' as the inside become completly clean silver metal again..
https://www.hihouse.co.uk/dr-magic-o...l-cleaner.html