I would think that they go to the differential pressure sensor Richard. My Range Rover has the same set up and the soot level is calculated by measuring the DP across the DPF to decide when to regenerate . The ash is usually what's left of the soot after a regen but I'm surprised that there's enough of it to block the pipes. Maybe it's due for a clean out. There are usually local businesses that deal in this sort of thing. Maybe worth making some enquiries. If the pipe is leading to the DP sensor it would certainly bring the light up.
Good luck.
Pete