So, we've removed and cleaned the DPF. It turns out its fitted with a Euro Car Parts filter. When we ran the engine up after refitting, the DPF pressure was down to 0.4 KPa at idle. It then allowed us to carry out a static regen but the car still had a slight misfire. On further investigation, it turned out that No. 4 cylinder injector had two washers under the seat. Removed second washer and the car now runs and drives perfectly.

After rechecking fault codes when road tested, I've noticed that code P2458 DPF regen duration has returned. I don't even know why its trying to regenerate the DPF when its just been done unless the pressure is high?