As said if there is a dpf related code in the ecu the ecu will abort any kind of forced regeneration as will a high KPA reading from the second sensor.
If the dpf differential pressure sensor hose has snapped its usually on the plastic part of the pipe around the steering rack area.
If you take the car for a run once you have flushed out the DPF the coolant needs to be at 80 degrees or above. Since the zafira b does not have a temperature gauge you can access it via the radio by doing the following.
Radio on
Press and hold settings till a beep is heard
Depending on the radio display type there is two different ways of getting it to show.
If it has the small two line display (tid) screen press the bullseye button a few times till you see a number with OT and another number next to it with a V so it will look like 80 OT 0V
The number next to the OT is the operating temperature.
If you have a higher series screen.(BID)/(GID) etc press the BC button on the radio till the same os displayed on the screen.
For a regeneration to take place on the move the engine speed must not drop below 2500rrpm As this will keep the dpf nice and hot and get to its target temperature of approx 620+ degrees.
If you have a trip computer set.it to the instant reading as this will most likely indicate the regeneration is taking place the the reading will drop quite a bit whilst its doing it.
Once done you may see a few puffs of smoke from the exhaust.

A forced static regen runs the engine upto 3600rpm and can sit there for upto.an hour. When the actual cleaning process is taking place doing it this wah a audible change in the tone of the engine can be heard.