An update:

First the good news - the ECU started its own DPF regen during a drive...
Then the bad news - it was just 54km after the previous regen...

I was driving around with BimmerTool connected. Exhaust temp around 350°C and Water temp 90°c. Soot rose quickly from 25g and at 50g the ECU began a regen which completed successfully some time later. Then after just a few kms, soot was back up to 40g.

My thoughts...
1) Soot mass is a function of DPF exhaust pressure so either the DPF is truly blocked or the Pressure sensor is reading falsely. Or there is a problem with the interconnecting hose. Looking at the BMW parts diagram, it is not a differential sensor so unlikely the hose is a problem as a leak would decrease the pressure.
2) I deleted the Swirl Flaps a few thousand miles ago, so I'll re-instate them to rule them out...
3) Car has a power-tune mod by the previous owner. I don't know how much that is contributing.

Next steps...
1) Vacuum hoses replacement was on the cards so I'll do that at the same time as replacing the swirl-flaps. I'll also change the pressure sensor.
2) I'll replace the DPF if the hoses and pressure sensor replacement doesn't work...

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Early 2010 E70 3.5D XDrive Msport