I have now replaced the pressure sensor and physically measured the back pressure and there basically isn't any - or very little. So I reset the pressure sensor learnt values, reset the DPF again and road tested. After 16 miles the DPF is up to 8%

I have tried to regen the DPF but I appear to be experiencing some issues with my Tech 2 at the moment as it keeps loosing comms with the car which it did part way through the regen process. I'm very worried about trying it again in case I damage the new DPF!