Quite a few conditions need to be met before it will do so: e.g. engine running temp >70 deg C for more than a couple of mins; no DTCs (or at least no DPF DTCs in ECU?); dpf loading above a certain threshold (70%+?); constant(ish) & minimum speeds; exhaust temperatures sustained above (and below) certain limits (less than 1000 deg I think?). I don't think this is the case for Vauxhalls but in some cases you need the correct quantity of fuel (probably around 3/4 tank). Try and find the exact criteria for your vehicle if you can. Essentially the info is similar to the sort of lists you encounter when trying to find the criteria to enable obd monitors.
Also check for other basics - has the intake been smoked? Gently ask the owner if the engine has been remapped. Either could cause excess smoke.
With respect to the differential pressure sensor I think these are sometimes used to instigate regen but not always. Sometimes they are used to map the system in some way (don't ask me how!) and/or perform straightforward diagnostic functions.
Finally remember also that static regen is not as good as dynamic regen and that you should make sure it is going in to dynamic regen mode on a run after a static regen. Sometimes a static regen won't get the counters reset below 70% so you have to take it on a dynamic regen run straight after. Also if you are instigating a regen with your scanner (either static or dynamic) then it could chuck itself out of regen mode if any of the above conditions (e.g. temps too high/low or dtcs appear) are no longer valid.
HTH.
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