Just found this post on pistonheads about the dpf regen, worth a read.
Those that keep having the regen occuring at odd times may need a new sensor?
Generally the way it works is the cars DPF pressure sensor tells the ECU how blocked the DPF is when it get slightly clogged it does a sort of lower level (passive) self regen and holds the revs up a bit and has generally slightly poor throttle responce, almost feels like the throttle sticks sometime.
If sensor and ECU deem its proper blocked, it will put the DPF warning light on the dash, then the car is requesting you drive it in the DPF regen style, (see bottom). This is full on active regen the throttle will be crap and engine may smell very hot while you do this (dont panic this is normal) If you condinue to drive without regening with the DPF light on, eventualy the check engine light will come on and you will have to take it to the dealer. The pressure sensors are a bit rubbish and fail from time to time and confuse the F**k out of the ECU making it regen to often or not noticing that you have done the regen correctly when requested by the car and still putting the check light on as if you havnt done anything about it.
In the event of a sudden change in the car, seemingly regening all the time, when it didnt before, could point to sensor failure. If its periodical every 500-1000 miles, then its the normal lower level regen happening.
How to regen. This will be most effective when the car is in regen mode, either you have noticed it doing it passivly or the DPF light is on.
Regen driving style, light to medium load higher rpm is the key, dont just thrash it to the red line thats almost as bad as pottering about. Go across the gate with the gear selector put it in a gear where the engine is doing 3000-3500 rpm and just maintain speed. If your doing 70 on a main road thats great, just do it in 4th instead of 6th. If your doing 50 on an A road do it in 3rd. 20 mins like this, with one or two slow downs and speed ups with full throttle helps clear out the EGR at the same time.