Regen with smoke

Hi all,

This has happened a few times now,

When the car is doing a regen as it comes to an end as I slow down into a 30mph limit I get 1 huge plume of smoke out the back. 
only does it once per regen not like when when erg valve sticks and you get smoke every time you let off throttle.

Not long serviced the car. Recently done timing belt. ( did it before belt change) 

46000 miles. 
not using oil 

soot level is about 37% in dpf. 
2L Diesel  vrs 67 plate 

any ideas