Hopefully this will help someone in the same predicament. It appears that the sensor reading -40C was not the issue. I don’t know if I made a mistake, and maybe one of the 4 temperature sensors is listed as something else in the PIDs. But it doesn’t seem to be stopping the regen. And it definitely has 4 sensors.

Problem turned out to be a blocked fuel vaporiser unit. Tried DPF cleaner down the hole, heating with Mapp gas and blowing with compressed air repeatedly, no success.

It can be accessed from the top, removing battery box, and airbox. Being careful to take note of attached cables. The fuel line is delicate, be careful.

It was a nightmare to remove, Crow’s foot spanner, O2 sensor style sockets, more heat, tons of penetrating oil overnight etc. couldn’t do it.

In the end I had to cut the fuel line and cable, to the point of no return, put a six sided socket and breaker bar and eventually got it out- and very luckily without taking any threads with it.

Pressures dropped, closed loop soot dropped as low as 5%, open loop 23% and the number of consecutive failed regens reset to zero. Plus no error codes.

PS- even if it comes out without threads, due to the carbon, I would recommend using a thread chaser to clean the threads and make fitting the new one much easier. They can be had on Amazon for around a tenner.

Appreciate everyone’s input and advice

Thanks for that detail - vaporiser check is always my go to suggestion if there are no other stupidities in Forscan.